I'm certainly not the most experienced individual in the world, but I've been playing in a jazz combo of some form or another for the past 5 years, and a bit over a year ago I started up the group I'm currently with, Zarathustra. (If you want to check us out and see a little bit more of what we do you can send me a private message.) I'd say that most of the stuff that people have been saying is valuable. I think there is nothing better in terms of learning how to play than actually playing in a real, live setting.
Now onto the important stuff:
1. Make sure you've got access to music. This may be through a teacher, parent, friend, whatever, but get some fakebooks. Once you have some fakebooks, if you have any knowledge about music theory then do some arranging and make your own arrangements. I'd say get comfortable playing some standards before you try writing your own stuff (and when I say some I mean a lot.)
2. Try your best to practice as often as possible. My group performed at least once a week and practiced at least once a week (provided we didn't have any other gigs set up) and we learned a lot rather quickly, so imagine what you could do just by practicing twice a week. And make sure these practice sections are actually practice sessions. It's good to occasionally go off and jam on whatever you may like, but it's just as important to spend time woodshedding even the most basic of jazz charts.
3. Look around for local places that my be good performance spots and get to know the people that work there. Our first gigs came from a local coffee shop in our area. Myself and the other "leader" got to the know the owners of the place well enough (or at least became comfortable enough with them) to ask them if they wanted a band to play there and that's how we got our start. And don't be afraid to ask multiple places, because as long as you bring in customer's, most places will be more than willing to let you play there.
4. Use any connections you might have. I know some very great musicians in the Northwestern Ohio/ Southern Michigan jazz scene and thanks to them I've either been able to pulled down a gig, get music or have a good word put in on my behalf. If you know people with connections, ask for their help, chances are they will be more than willing to help you.
5. Make sure there is a "leader." That leader doesn't need to take any public credit or get on an ego trip about it, but somebody needs to be in charge, run rehearsals, book gigs, etc etc. Without that things get muddy and nothing gets done. My group actually has two leaders. This works ok because myself and the other leader communicate everything as soon as we find out about it, but this can also lead to trouble, so I'd suggest just picking one and going from there.
6. Be professional. Seriously. It's important. Show up on time, be set up WELL before your show starts and be dressed appropriately for the performance at hand. If you're playing at a bar on a friday night you could get away with looking a bit casual, but don't ever look bad or sloppy, look clean and nice, even when "dressing down." Act like you know what you're doing (but don't be cocky) and do your best to be polite to everybody, even hecklers.
7. Have fun. A big part of your success lies in 2 things: 1. whether or not the group is committed to learning and working on the music and 2. whether or not the group enjoys playing together. If both of these things don't happen then you'll stumble the entire way. I've actually had to remove people from my group because of both of these problems. It's never fun to do that, but now thanks to that we are a hard working, cohesive group that loves what we do. If you guys have fun while you're playing not only will you give off a more intense and full performance but the audience will be able to tell that you're enjoying it as much as they are.
I suppose that's all I have at the moment...but I'll let you know if I think of anything else.