Well, the simplest way to break it down would be to look at the three primary colors of a rock band: drums, bass, and guitar. In terms of instrumentalists, these are the most basic and common positions.
The drummer would be the timekeeper of the band, by virtue of being the most rhythmic player. If you have a natural flair for rhythm (or just a fetish for hitting things HARD), then this position is probably the one for you.
The bassist is the bridging member of the band: what you effectively do is connect the rhythmic section and the melodic section together. If you're a complete newbie to rock music, the bass is responsible for the low notes you hear in between the guitars and the drums. Some people look down on bass cus its not as glamorous an instrument as guitar or drums, but it's very important in the context of a band because it keeps the bottom end movin'. A good bassist will be able to lock with the drums while still effectively supporting the guitars.
The guitar is, of course, the most well known instrument in rock music. Most guitarists are split between lead and rhythm: the lead guitarist plays melodic lines (the ones that sing in your head), while the rhythm guitarist is responsible for bringing out the chord progression and making things headbangable. Of course, sometimes the distinction between lead and rhythm is hard to tell, especially in a band like The Rolling Stones, where they often swop. But that's the most basic division.
Of course, if you're interested in singing, you could always be the vocalist. The vocalist can often be an instrumentalist or just a pure singer. Most of the time, regardless of whether you're playing or not, the vocalist is responsible for lyrics and crowd interaction as well, by virtue of the fact that you have the mike! if you love singing and/or/ are a buay hiao bai person, you'd be good on vocals.
Hope that helps.