First band I ever tried out for I knew just a few basic open chords and one rhythm pattern. I thought I did good, but I found out later that they were laughing their butts off at me after I left.
Second band I tried out for, I only knew a few more chords and rhythm patterns and after hearing me they turned their backs on me and ignored me for the rest of the session.
Third band I tried out for just needed a bass player cause they were playing a gig at a hotel in town. I rented a bass and showed up but was so bad that somebody from the audience actually got up and replaced me. (still made $150 though :)
Next band was a country band and luckily the basic chords I knew were perfect cause I got hired & went on the road for a year. Hated every minute of it, but learned a whole lot about performance, playing in a band and having fun with groupies.

I'd basically advise you to start as soon as possible. The more you try out and the more you force yourself to play, the more you'll learn and the better you'll get. Definitely find out what cover songs bands are doing though, and learn as many of em as you can. The skill level doesn't matter so much as knowing songs to play.