Most bands want 2 things: musical competence, and a good personality match. a Professional attitude (show up on time, help out, keep your commitments,etc ) helps too.
My audition experiences while trying out for various original bands:
1.) find the ad on craig's list. If i like what I read:
2.) got to their myspace page and listen. If I like what I hear:
3.) contact them and arrange an audition for the next week or so
4.) rip their songs from myspace to my ipod and learn them as best I can (usually writing out a chord chart, maybe some notation)
then I show up with my charts and my bass and go for it. after that Its not up to me
Also, if you're just starting,go for the most skilled group that will have you -it's an educational gold mine to play with superior musicians.