actually to play/compose jazz you have to have a more complex understanding of harmony and rhythm
HERE you'll find some theory that might help
HERE also
those are theory and harmony generally speaking
if you gonna play in a band there are some rules to follow:
1. Everyone should play the same piece.
2. Observe the repeat signs only if what you just played was interesting.
3. If you play a wrong note, glare at one of the other players.
:))) took those from actually a great site about JAZZ IMPROVISATION
also, playbg impro's on PLAYALONGS might help
the most important is to listen a lot of music, that enters into your brain and turns you into a jazz zombie like me :p
cheers