#1
ok so i know how to find out what key a chord progression is in

but is there a way to find out what chord progression to play over a key
#2
Just take any note from the scale and build a chord from it by adding the 3rd and the 5th. For example, here's all the chords for the C Major scale.

I- C major (C, E, G)
II- D minor (D, F, A)
III- E minor (E, G, B)
IV- F major (F, A, C)
V- G major (G, B, D)
VI- A minor (A, C, E)
VII- B diminished (B, D, F)

Some basic chord progressions are (I, IV, V, I) or (I, VI, II, V, I).
#3
thanks because i couldn't figure out how you would go about doing that