I started trying to play jazz guitar pretty recently too. I'm not very good at it at all yet.
Here's a site that's not bad:
Jazz Guitar Online
Here is something I posted in another thread on jazz that may be of some interest:
Check out this site:
Jazz Theory
For guitarists, the best book I've found to start with is 'Jazz Guitar' in the Hal Leonard series. It explains things really well.
A really important concept in jazz is chord substitution. The article in the link there looks really complex, but at heart it's quite a simple process, as long as you're good at chord spellings.
It revolves around the fact that many chords are similar or rearranged versions of each other. The simplest one is C major, which can be thought of as A min 7 with no root.
Simple substitutions involve extending the basic chords you're working with. So, instead of C major, you could play C major 7, C major 9, 11, 13, or any combination of these with raised or flattened notes depending on what sound you want. Since there are so many possibilities, it's best to start off simply and build up your vocabulary of chord progressions.
Another good sub is to take any seventh chord, and then substitute it with a half-diminished chord a major third higher. Example:
For A7, an obvious sub to go for would be A9. However, you could also use C# min 7 flat 5, which is an A9 with no root. The bass player will probably play the root, so you can get away without it.
The reason why you play these chords is to give your music variety and to take advantage of the huge possibilities that harmony has to offer. Playing in a jazz group and sticking to basic chord sequences with no variation would sound dull after a while, and very un-jazz-like.
As always with music, the key thing is to practise and keep training your ears.
