Originally Posted by: WashburndLets say that the song is Am Em and G in standard tuning.

Did you look at the tutorials on basics of improv?

In those you will get the basic idea of how to approach it. Write down all the notes in all the chords you are using. List them in alphabetic order. Do they form a scale? Use that scale and target chord tones as they occur in the chord progression.

All that is explicitly dealt with in detail in those tutorials.