A lot to unpack here. Keyboards are difficult instruments to play, and it'll take time. So, first off, don't buy the most expensive keyboard. A cheap one will do for now.
1) ALWAYS practice with proper form. Keep your back straight with good posture, and always have curvature in your fingers. My parents used to say that I had dinosaur eggs under my fingers that would be crushed if I played with straight fingers. Of course, the kid me couldn't allow that to happen;
2) You'll probably get the most benefit out of being able to follow chord changes as opposed to developing technical mastery, so study the circle of fifths, and be able to play the chords. Then, practice cadences to cement the chords and most common chord changes of 1 sharp or flat up or down;
3) Play scales in whatever style of music you want to play, e.g. major and minor, blues, etc.;
4) Try to play songs you like either by transcribing them for keyboard or playing the exact notes if there's a keyboard part in the song. The more you do this, the more instinctively you'll be able to play by ear.
I hope this helps, and please give updates on your progress!