Question for you guys...any help very much appreciated!

Ok So i'm trying to learn scales/modes/improvising....basically all the theory and technique behind guitar without taking the expensive lessons that i cant afford, so i've come to you guys for help! =D

Now my question as of right now that im curious to kno is...lets say you're playing a chord progression, A G C D A and i wanted to toss a riff in there over it (my friends much better than me but he always wants me to jam but i cant do any sort of basic improv, the fretboard is basically anarchy to me at this point)

But anyway, if he's playing that progression, to improvise over it, im guessing i would revert back to a scale? correct? Now how do I kno which scale to use? Would I be able to use any A Major scale since it starts and ends on A Major? or how do i kno which scale to use? if in fact thats what id be doing, which im pretty sure it is?

thanx guys!

Do what sounds good. Theres no rules, the fretboard is yours to explore!

That's going to be a weird riff, because he is changing keys. I'd have to hear it, but I would suggest playing in A major on the first chord, then playing in G major for the 2, 3 and 4th chords, then lead that back into the A major on the last chord.

Just experiment around. I think there are a few good listens on modes here, and that would be a great place to start for a riff like that.

Is he playing A G C D A chords, or A G C D A power chords?

That might make more sense. In which case my person inclination would be for an A minor pentatonic. But the door is wide open if they're power chords.

Scales and chords good sir. I'm going to put up a lesson soon.