Basic Music TheoryMar 25 — By The Tweak
...And I mean, Really Basic!
Find Middle C. Its the white note to the left of the two grouped black notes in the middle of the keyboard.
Count the white notes going p from middle C like you sing your ABCs. C D E F G A B C <- That is the Major C scale.
Now find the A and count up, all white notes. A B C D E F G <- That is the A minor scale.
That is the start to learning keyboard theory, and if you just learn that, you can use the transpose buttons on your keyboard to play in any major or minor key.
Its a great Idea to get a grease pencil (which can be rubbed off without marring the keys) and write directly on the keys.
Lets say you want to play a piece in a hard key, like Eb minor. You would first find the scale, then place a little piece of tape on each key in the scale. That way when you are looking for leads and basslines and chords to "fit" your song, you can restrict your playing to just the keys with tape on them.
Here are some books to help get you started: