While it is good to learn theory, you can also jam without any knowledge of it. Here's a few easy ones:
G-D-C: Knockin' on Heaven's Door
G-D-Em-C: All I Can Do is Write it in a Song (Skynard)
C-G-D-A-E: Hey Joe
I also like playing D-Em over and over with an occasional C or G mixed in. A-D sounds good too--throw in an occasional E and you've got a three chord rock/folk song.
Oh, and don't forget the versatile combination of C-Am-F-G (Pink Floyd's "Thin Ice" and Dwight Yokum's "Thousand Miles from Nowhere" and Tom Waits' "Cold Cold Ground"). All these songs start with a few bars of C-Am, then go to a C-F-G progression, then resolve back to the C-Am.
Then there's E-A-B, blues progressions. Try playing E-A, E-A, E-B-E. I like playin' the blues like this, too: A-A-A-A, D-D, A-A, E-D-A-E, repeat. You get the picture. The combinations are endless. Make up your own. Have fun!
DC
[Edited by David C on 07-17-2002 at 08:54 AM]
"It's all right son . . . we told you what to dream"