These other guys have pretty much nailed it.
You really need to practise reading music as much as possible. Don't just think about notes either, there's also alot of complicated, syncopated rythms in jazz charts.
Listen to Jazz. There are alot of styles that relatively "standard" basslines & chord progressions. It is also possible (depending on the skill level of the group) that you will be given charts that don't have a written line. These charts will just give you chord progressions - no notes, you will have to rely on your knowledge of chords and your knowledge of that particular style to create a bassline.
Soloing - Soloing isn't something I would focus on at first. If you get this other stuff down, soloing will come on it's own. If your struggling with this other stuff, your director isn't going to throw a solo at you (at least not in a performance situation).
Don't forget the basics - You will probably (again, depending on the skill of the group) going to run into a jazz chart at some point that will challenge your chops. Your technique needs to be top notch for jazz. You need to have good technique (proper thumb position, good use of all four fingers, etc.) becuase there will be ackward jumps, big stretches, and fast lines. Don't forget endurance too.
Your also going to need to learn to focus on alot of things besides your bass while your playing. You will keep and eye on the conductor, keep an eye on your music, and most importantly - you will need to be able to listen and react to what the ensemble is doing.
Good time - Your also need to have a good feel for tempo. Try practicing with a metronome. Nothing's worse than a bassist or drummer that is always trying to speed up or slow down.
Time signatures - If your not familiar with time signatures besides 4/4, you should check them out.
Is this a high school jazz ensemble? College? Other?