Like flipflop104 says, what do you want from your night?
There are lots of different nights to suit a range of different tastes. There are the clubs which have cheesey music, quite a lot of locals, and are pretty much like any other club of that kind. These include: Creation, Majestyk, Evolution, Oceana. From personal experience, don't go to Evolution unless you want a fight!!
Other clubs, such as Mint, Rehab, Space offer a more "clubbing" experience, with a good range of DJs.
Bar wise, there are loads to choose from...the Corn Exchange area in the City centre is a good place to start. There are also some bars in Headingley, which is just outside the city centre, apprximately �5 in a cab. That's the more studenty area, but on the weekends its just like any where else to be honest.