Basic Chord Progression Formula

Key of C

I: C
II: Dm
III: Em
IV: F
V: G
VI: Am
VII: Bdim

Chord Position (In Progression) ----> Suggested Chord (Relative to Key)

1. ---> any chord 
2. ---> V7
3. ---> VI
4. ---> I or V7
5. ---> I
6. ---> II