I'm starting to learn to play the piano by myself (I've had experiences with other instruments so I have some sort of musical framework), and I would like to know what technical nuances I have to know so that I won't start playing the wrong way.
- The biggest mistake that many starting-out (and professional) keyboardists, including pianists, make is sitting at the piano incorrectly which results in playing with tension. Make sure you learn everything you can about correct bench height (generally, your forearms should be parallel with the keybed), sitting in a relaxed manner and relieving all muscle tension in your hands, head, arms, chest, legs, back, and wrists. Learning bad posture habits cause tension, which makes it more difficult to actually play the piano, and may potentially cause an injury later on. Learning good habits will enable you to be less tense mentally, which will facilitate faster music learning, better sound, and more easily attained technical ability. There are many myths out there about positioning at the piano.
- Another mistake many beginner pianists make is not practicing every day. Perhaps take a day or two off a week (not in succession), but when you practice, you are programming your brain with short-term information. If you skip a day, you lose a good deal of that information, which can be difficult to regain. Also, if you take lessons with an instructor, try to have lessons every week instead of biweekly. It will encourage you to make faster progress.
- Many pianists, when learning their first pieces, do not establish good practice habits. This is also linked to tension. If you find yourself making many mistakes during the course of the early stages of learning a piece, you could be practicing incorrectly. It only takes a few runs through the piece, or perhaps even only an especial troublesome passage, at a slower tempo to solve issues for good. Also, make sure you know and employ all the other important practice techniques, like practicing hands or voices separately; listening to a recording of yourself playing; or trying several tempos to find the best one for practice and performance.
Some final advice, also related to tension, which as you may have guessed is very important: Seek beauty, not perfection. When you make beautiful music, perfection gets thrown in. What I mean is, trying too hard is not beneficial; much better to take it back a notch, think about and examine the music closely, take a break, spend time with the music away from the keyboard, and of course practice things slowly, with relish, so as to observe and learn every musical instance. Then learning the piano will be enjoyable, and it won't be a breeze, but it will be easier and you will play better.
Playing too fast too early is the most common mistake I know. It leads to bad habits of movement: your fingers don't move enough, because they are to weak and then the whole hand and arm "helps out" but they cannot keep the velocity for long time, so they start to cramp. Which of course influences breathing and blood circulation in a negative way, and so on...
You surely know this already from your other instruments: practice slow and the fingers will learn "in the background". When feeling bored, look for details to (deeper) concentrate on.
In the case of the piano(forte), you may look at
- a wide range of dynamic - try to get a sure sense for velocity
- no control over timbre while the tone is "ON", concentrate on its boundaries, so
- be precise in attack timing (even meter, balanced tempo), also simultaneity of the hands
- get a feeling for releasing the keys, which makes also a kind of (semantic) timbre
This indeed does seem fairly broad, and my answer might not be the most relevant to someone just starting the piano and is based mostly on personal experience. I also have the feeling that a lot of issues are specific to the player - what is an issue for me might not be an issue for someone else, or at least not to the same extent.
In general, at least for me, most piano-specific issues (i.e. not related instead to, for instance, rhythm) I've had are generally linked to various forms of poor control.
For instance, I've had issues with smoothly controlling fingers 4-5 in various passages (usually somewhat faster ones) to allow for evenness. A somewhat similar issue also crops up when playing harmonic intervals sometimes, on the same fingers, as (at least for me) they seem somewhat weaker.
There's also the issue of positioning the rest of the arm properly - some "positions" (dependent on the specific passages) of the wrist/arm can lead to straining and discomfort, not to mention subpar playing. (This generally also seems to be linked somewhat to shoulders that are too tight.) This, however, seems to be a somewhat "general" issue that can occur pretty often - as mentioned, whether or not a specific positioning of the wrist/arm is going to be problematic depends a lot on the specific passage and the technical requirements of said passage. It does, however, tend to be more of an issue for me in fast passages, but beyond that I really can't get more specific.
Finally, there's the issue of excessive pedaling, which can crop up pretty frequently for some people.
It's been said in a comment, but deserves a fully-fledged reply: your biggest mistake is thinking you can do this without a teacher. Even if it's only a consultation lesson now and again - that's better than nothing.
Note the number of people who have tried to give you a piano lesson in their replies!