Shower at least once a day. You don't have to wash your hair once a day (more on that later), but you should clean your body that often. Already had your daily shower, but you got sweaty working out/walking home/going to your job? Shower again.
Wash your hair at least every other day. How often you wash your hair depends on your hair type. If it tends to be coarse and dry, you can probably get away with washing every other day (and it will actually look better than it would if you washed every day). If it's thin and silky, though, you might want to stick to washing every day.