| If at all possible, record your rehearsals and play the tape back many times at home. You don't need a fancy recorder. Just get a simple $20 one at Radio Shack. You are not looking for tremendous fidelity of sound. Instead, you are listening for how you sound with the band, how you sound with the drums, what the music sounds like (Which will be easier if the band plays covers. If they play originals, the tape will help you memorize the songs much quicker.)
Keep the tapes and date them. You will notice progress as you attend more rehearsals. You will sound more confident and lock in better with the band.
If the band can help you with what their previous bassist played . all the better. Don't be surprised if they can't, though. Without knowing the particular band, they may not be able to even tell you the key they are playing in, much less the chord progression. I have been in bands like that.
If you are lucky,though, they may even have chord charts and that would be an infinite help to you.
By the way, good luck. There is nothing like playing in a band to speed up your learning and development as a bass player.
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"Jazz sounds like a very good blues band that fell down a flight of stairs."
Michael Buble, Canadian standards singer | 
. Not implying you are stupid. Just keep the groove at first. Don't try to do all you're cool fills or runs right off... be reserved, no one likes a hot head bassists... thats why God made guitarists 
