Our band is playing desperatley wanting by Better than ezra. The chords during the verse are F, Am and G. What can I play to add a little fill to the song(what scales. etc.) I want to learn how this stuff works!

Are you telling me you don't know the major/minor scales ? Anyway, with these simple chords, you could play a bunch of scales with each chords. Basically, on maj chords, any scale that doesn't have the third flatted/sharped when you start on the root, and on the min chord, any scale that has the third flatted. With both, you need a perfect fifth in the scale (not aug,dim). The octave need not to be flatted/sharped, but I don't know of a lot of scales that have the octave altered. With this in mind, the possibilities are endless.

Or you could just riff in a key that has these notes comprised in them: ie. C. The latter approach is simpler and quite sufficient for rock fills, especially since F and G are maj in the key of C.

Then, when you know all the modes and scales (and what keys comprise what note), you could take each individual chord and change key, making it the root and harmonize by playing, let's say, a minor sounding mode that is comprise in this key. A lot of harmonizing is done on the third, fifth, seventh or octave of a chord.

Since Amin is the relative minor of C, you could play a natural minor in A all over the neck during this chord, or simply C Ionian or G Dorian.