I know you have been looking for something like this for a while — something that would silence your doubts, give you answers for your questions, confirm your hopeful guesses and give you the final little push to go for it: to start learning to play an instrument — from scratch — as an adult.
So here it is:
1) Choose an instrument you really want to play
This might seem obvious but you would be surprised if you knew how many adults rationalize their choices. Sure, sometimes compromises are inevitable, and the practical aspects of our daily lives cannot be ignored completely. But imagine a situation like this: you always loved Miles Davis, always wanted to play the trumpet like him, and it seems that at last you can turn your dream into something real. However, your apartment is small and acoustically completely unsuitable — you know that your neighbors are going to hear every note you play, and their patience will probably end quite soon. So instead of getting a trumpet you go for a guitar. Do you think that after picking up an instrument only because it's more affordable or sounds softer you are going to have enough motivation to practice? Whatever the reasons that hold you back from choosing the instrument that truly fascinates you, do not just give in. Do your best to remove or diminish any obstacles — maybe you could improve the acoustics of your apartment, talk to your housemates to find out when you could practice without disturbing anybody, or get the instrument you want second-hand?
There really is something like "the right instrument" (or instruments) for you. If you do not feel excited when thinking of playing a specific instrument, there is little hope you are going to stick with it and get really good. Get rid of what holds you back and get excited, because you are going to play the coolest instrument ever! :)
2) Buy a good quality beginner instrument
Even if you are on a budget and not sure whether you will be able to stick to playing the chosen instrument for years, do not buy the cheapest instrument you can find. And certainly avoid buying it online. Instead, save up some money for a good quality beginners' model. Go to a good music store or even more specifically, to a store / reparation service that specializes in the type of instrument chosen by you (for example, just pianos, or string instruments). If it means a one-day trip to a city, do it.
There are a few reasons why you should not buy the cheapest instrument online.
First of all, you want to choose yourself an instrument that feels and looks like something just for you. If you have absolutely no idea how to play it, you can ask somebody who plays the instrument to come with you — for example a teacher, or a friend. You can also ask the the seller about the differences between various models they offer. A good instrument makes it easier for you to start, and you can be sure that when any technical problems occur, you can get professional support. And in the unlikely case of dropping your hobby after a while, selling your instrument is going to be considerably easier — sometimes even the very shop where you purchased your instrument might help you with that. So, in a nutshell, save up, go to a professional music seller, and choose yourself.
3) Create a suitable practice space
Whether you live in a tiny apartment with your family, share an apartment with a few housemates, have oversensitive neighbors, or live in a huge manor on your own, creating a suitable practice space can be the factor that decides about your success (= persistence) or failure (= giving up). The best practice space:
gives you some privacy,
is suitable acoustically,
is spacious enough for you to practice comfortably, with optimal posture,
does not distract you / helps you to focus on the music.
This does not mean that you need have a separate room just for your music activities. If you can afford this luxury, go for it, if not, there is no reason to worry. Your practice space can be a moveable space created easily by putting together all you need. You can practice in your bedroom, or in the kitchen, or even in the bathroom (if the sound rebounding from the tiled surfaces is not deafening, of course). It can be actually very useful to practice in various places because it allows you to experience various acoustic environments, train your attention and your ears. However small or spacious your "practice corner" is, remember that proper lighting, enough room for setting up your "gear", and - if possible - closed doors and windows can increase your comfort of practice. They will also help you to focus and encourage you to experiment and try things out without worrying that someone is going to hear all those out-of-tune or squeaky sounds you produce.
4) Get the basic gear you need
Be it a proper stool for your piano, a music stand, a mirror to check your posture, or some handbooks and scores, make sure that you get all the basics organized as soon as possible. You do not want to get stuck during your practice just because you cannot tune properly, have no idea what piece to learn next or cannot check your tuning or tempo. You also do not want to run the risk of twisting your body awkwardly to read music (which happens for example when you put your score on a table instead of a music stand). Get whatever you need to make your practice easier, smoother, more fun and more comfortable.
5) Schedule time for your practice
Be optimistically realistic: there probably are days during your week when you know for sure that you will not have time or energy for playing the instrument. Without pushing yourself, make sure that you can find time for practice often enough — 6 times per week would be absolutely awesome, but I am aware that this option is very optimistic. However, aim for 4 practice sessions per week.
Remember that playing an instrument requires developing specific muscle memory — so, like in sports, to shape your body and stay fit 3 sessions per week are a must.
This is not to say that with less practice than that you cannot learn to play an instrument. You can, but your progress is going to be considerably slower, and as you certainly know, little progress can be hugely discouraging. So if you are already doing this, play as often as you can.
Use reminders like sticky notes, calendar entries, or even just leave your instrument on the ready so you do not need much preparation.
6) Stay fit to boost your progress
Whichever instrument you have chosen, music practice alone is very rarely sufficient for developing the muscles you need to play well and enjoy it. If you spend most of your days sitting in front of a computer, go everywhere by car and are not a fan of a sporty lifestyle, playing some instruments may turn out to be quite exhausting.
At the beginning it is normal that you feel some muscles getting active and a bit tired. But playing an instrument can be physically demanding, especially when you spend increasingly more time playing, with the music becoming more complex. There is one simple thing you can do to become good quicker and without feeling tired: get fit!
Enrich your lifestyle with some physical exercise — walk whenever you can instead of driving, choose stairs over a lift, do some yoga to strengthen your core muscles and stretch yourself, work out at home with one of fitness YouTube stars, join a gym or enroll for a dance course. There are so many ways of getting and staying fit! Choose something that you find enjoyable and easy to add to your lifestyle. If you are not a sporty person, it is fine — you do not need to be sporty to play an instrument. But the more balanced and the stronger your body is, the easier your music practice is going to be.
7) Listen & watch
Do you realize that you actually can learn from the best performers? And that you can be your best teacher? To make it happen, train your ears and your eyes.
Listen to recordings of the pieces you are working on, to standard music literature for your instrument, to a variety of styles and genres. You want your ears to get familiar with the sounds you are going to produce. And you also want your eyes to get familiar with what it looks like when one plays, which makes it easier to find out how you can use your body optimally. This helps you also to recognize whether your practice brings you towards your desired results or not.
Listen to solo pieces, to music for bands and ensembles where your instrument appears in order to understand better what roles your instrument usually plays, what tasks it gets, and what the proverbial icing on the cake is — tricky stuff, unique techniques and really cool sounds.
Go to concerts and gigs and observe professional musicians — everyone has a different style and not everything is going to work for you, but watch and try out whatever looks natural, comfortable, and cool!
If you listen to other learners, focus on the ones that are more advanced than you. As listening to or watching something stimulates your brain in specific ways, you are naturally inclined to act similarly to whatever you concentrate on. Even if listening to a world-class performer might make you think that you are hopeless (you are not! getting better just takes time and effort), stick to it and learn from the best.
By the way, have you ever noticed that when using a foreign language, it is easier to improve your skills when you deal with native speakers rather than other learners? The same is true for music — find your "native speakers" in music and learn from them :)
8) Look & understand
No matter which instrument or music genre you are interested in, being able to read music gives you a huge advantage. If you can read music, you are more likely to quickly learn the basics (and more than that) of music theory — as a result you develop a deeper understanding of things like harmony, structure of pieces, arrangement, composition, and songwriting.
Learning music pieces with scores allows you to practice without even unpacking your instrument. Bored on the bus? — Look at your scores. Waiting for your boyfriend to get ready for a dinner out? — Look at your scores. Digesting your dinner and feeling too full and too tired to do anything? — Look at ... well, maybe in this case just chill out listening to something nice like listening to music or watching an episode of your favorite series :) You get the idea though.
Reading music — noticing all the important information that might easily get overlooked when you multitask (= play) can help you to learn a piece quicker and understand it better. You might also observe that when you read music, your fingers feel like moving, or you might visualize yourself playing the passage you are looking at. This is a great exercise as your mind processes imagined actions like real ones, so by reading scores you just get some extra practice done.
9) Work on your technique in a creative way
Repetition is good and necessary. Patterns are great. But be careful: never allow some scales or other technical exercises bore you!
If you repeat something, be aware of the reason why you repeat the thing in the first place. Do you want to play it differently? What needs to be improved? Or was what you just played exactly the way you want it to be, so you repeat it in order to remember the right "settings" better? Brainless repetition is a waste of time, nothing else. Repeating something with proper attention to details and with general awareness of the sounds and of how you use your body to produce them is your path to mastery :)
Technical exercises are most powerful when they go back to basics, when they are simple. If you use some music book with technical exercises based on scales, arpeggios, rhythmic and melodic patterns, try to memorize a chosen pattern and then play it in a varied way without looking at the score. For example, use your ears and knowledge to transpose melodic patterns into different keys rather than just reading music without thinking what you actually are playing. Be creative. Observe and listen. Playing a simple technical exercise with repetitions from a score kills your brain and makes it actually more difficult for you to stay focused — before you even notice it, you just play notes instead of making music. When you are attentive and creative, you enjoy music practice way more!
10) First find a learning buddy/teacher, then find a band/ensemble
As long as you are a beginner, you should have a teacher or at least a learning buddy — somebody more advanced than you, who would help you get rid of unwanted habits, and who would share with you their experience and tips.
Once you have learned the basics, try to find a band, an ensemble or any group of people that make music together. Of course, take into account your music preferences and your level, but do not shy away from auditions (usually even non-professional groups want to know who they take on board) just because you do not play like your favorite performers.
As soon as you can play in tune and rhythmically, as soon as you can keep playing even after making a mistake, you should reach out to other people who also play instruments and love music.
And this is it! I hope that this guide is helpful in some ways if you are planning to start or just started playing an instrument. Remember about these 10 golden rules and have fun! Good luck :)