If you eat complete junk all the time then maybe you should think about a multivitamin but chances are you're getting the proper nutrition if your diet is varied and somewhat balanced. Concentrate on how your diet looks for a whole week, not just one day.
As a woman, you should consider a calcium supplement since it is difficult to get 1200 milligrams in a day. Viactiv calcium chews are good.
The problem with most multivitams is that they're not slow release which means they are urinated out before they have time to do anything. Beware of vitamins that claim to have over 100% of your RDA - its a waste of money since you'll be peeing it away, and you could possibly risk toxicity if your diet is already high in some of those vitamins.
Do some research on the net - fitday.com is a good site for recording your food intake and they'll put together charts, etc., showing where you stand w/ vitamins and minerals.