an important addition to Maya_fanatics post is that when you have modeled half the teeth, and mirror them, then you should move and scale some of the teeth a bit so they dont seem to be too regular.
a good thing is to get a mirror and look at your own teeth.. also try to put your fingers in your mouth and feel how they are shaped aswell ..
You can either choose to extrude the gums from the mouth or model the mouth as a different object. I can't say wich is the best, cause that depends on the purpose of the model.
Senior 3D Artist,
Maya Software Manager & Maya Instructor
LEGO Systems A/S