Well, first off I am not the most knowledgeable person when it comes to theory but I would say the first three things you should know (in order of importance) are:

1. The name of [U]every[/U] note on the fretboard, theory is useless without knowing exactly what you are playing.

2.Intervals, simply the distance between two notes.

3.Chord theory, this will explain how chords are made, which helps in deciding which scale(s) to use when soloing.