Learn your chords most importantly.Learn scales and the degree of the notes in each scale. For example,if a song is in the key of E , here's the E scale E F G A B C D There are seven degrees here. Each degree has a value. The value of the first degree which is E will be major, the second will be minor, the third minor, the forth major, the fifth can be a dominant seventh I think, the sixth minor, and the seventh dominant seventh mabey. This is just a way to determine the type of chord you will use for each note in a song. There are many books on theory.Purchase one you can best understand. Hope this helps. God bless!