C - I
D - II
E - III
F - IV
G - V
A - VI
B - VII
C - VIII
Construct any major scale from your root note:
Whole, Whole, Half, Whole, Whole, Whole, Half
Or if you are from Europe and use semitones rather than whole step or half step:
Two, Two, One, Two, Two, Two, One
The basic rock and country chord pattern is I, IV, I, V. Using the C major scale above, that's C, F, C, G. If you do it in G, it's G, C, G, D. If you do it in D then it's D, A, D, G. If you can figure out WHY its GCGD or DADG to get I, IV, I, IV then you're doing great and well on your way.
Blues sometimes uses what's called a blues scale and flattens the third and sometimes also the seventh (which is a regular minor scale).
Good luck. Check into the theory forum on Chordie. It's amazing. Simply amazing reading in there from good teachers.
- Zurf