Hi, welcome to Pianostreet!A little more information would help with the responses to your questions. What are your goals in music? Do you want to become a professional musician, such as a performer, teacher, or composer? Or do you want to keep music as a hobby, while maintaining a similar standard of work to that of a professional?Piano technique: learn your scales, chords and arpeggios in all keys. These are the building blocks of which a good majority of repertoire is constructed. Set small reasonable goals, don't try to learn all this stuff in one day. For example: for a given week focus work on the scales, chords, and arpeggios for one major key and one minor key. The following week pick another set of major and minor keys. Also, don't spend your entire practice session doing only technical elements, they are important, but repertoire is more important. A majority of your practice time should be spent on music. Improvise music. It is never too early to start learning this skill. Make improvisation part of your daily practice routine. Use improvisation to explore the instrument, musical ideas, and technique. If you have not yet begun to improvise, just start. Achieving flow is more important at the beginning stages than attempting any specific technique. You will find an active improvisation community here at Pianostreet, just ask.Bach is always an excellent choice. Have a look at the 2 and 3 part Inventions.
in the school where i want to have my bachelor's degree (the only music school where i live) i have to pick an instrument, and the only plan of study is to become a professional interpreter
This sounds like a performance program, correct?A large part of studies in composition is studying repertoire and how other composers put together their music. Taking a performance program will serve you well in the context of composition, because you are undertaking that study of repertoire. What requirements does the school set for entrance into the program? Do you have all of these already? It sounds like you are preparing yourself to enter a Bachelors program. If so, can you list the things the school is looking for. It would help us point you in a more specific direction to prepare.
And that's it lol, those are all the requirements listed in the program.
From my perspective I find Bachs WTC overall more helpful in terms of how it develops you compared to the inventions.
Also, in which order would you recomend learning the keys?