As dogfish says. Personally I prefer a little more control than the auto settings, but they often give great results.
Image>adjust>levels.. and you should see a histogram type of picture, with three triangles underneath. Drag the outer two traingles until they are at the edges of underneath the actual histogram shape, and then drag the middle one round a bit until you're satisfied with the result. This is what autolevels does, but sometimes autolevels can mess it up a bit.
The adjust brightness/contrast as dogfish suggests. Sometimes better results than just using autolevels.