There are a lot of theories on this. Various methods work. Some people like a push, pull, legs routine. It goes something like this:
Monday: Chest, Shoulders, Triceps
Wednesday: Back, Biceps, Abs
Friday: Gluts, Quads, Hams
A full body workout 3 times per week is also a pretty good way to go. I did this with excellent results, although many people dislike this method.
Here's what I do:
Monday: Chest, Shoulders, Abs
Tuesday: Back, Traps, Abs
Thursday: Arms
Friday: Legs
These are just a few examples. There are tons of possible splits. A couple of general guidelines: don't lift three days in a row and leave at least one day between back and legs.
Try them all!