First off, the key to being a great mucisian and composer is knowing the musical scales. Then you should know how to read the key signature and notes in sheet music.
Assuming you know the name of the notes on the piano, here is what the musical staff looks like.
EGBDF are the lines, and FACE are the spaces.
------------------F------------------
E
-----------------D------------------
C
----------------- B-----------------
A
------------------G-----------------
F
------------------E-----------------
Obviously match each note you read with the note on the piano.
To read SHARPS in a key signature, look at the very last sharp. Whatever note that is on, go up to the next available line or space. That is your key signature.
To read FLATS in a key signature, look at the NEXT to the last flat. Whatever line or space that is on, that is your key signature.
After you know where the notes are on the staff and how to read what key the song is in, you need to know how to play it in that key.
Here are the keys in music ----
C Major, C Minor, C# Major, C# Minor
D Major, D Minor, D# Major, D# Minor
E Major, E Minor, E# Major, E# Minor
F Major, F Minor, F# Major, F# Minor
E Major, E Minor, E# Major, E# Minor
D Major, D Minor, D# Major, D# Minor
G Major, G Minor, G# Major, G# Minor
B Major, B Minor, B flat Major, B flat Minor
A Major, A Minor, A flat Major, A flat Minor
Each key is a scale. (Do, Re, Mi, Fa, So, La, Ti, Do) Knowing all the scales is important. Either purchase or google sheet music with all the scales. Practice reading and memorize each scale.
After you know all the scales, reading the key signature and reading and playing the song in the correct key will be a breeze.
Here's a link to some sheet music key signatures.
http://hymns.reactor-core.org/keysignatures.html