At the risk of oversimplifying things slightly, it's basically calories in vs. calories out.
This calculator designed for our age group: http://www.bcm.edu/cnrc/bodycomp/bmiz2.html
will tell you roughly how many calories you need to maintain your weight. (Start at lightly active, if you workout during the year, up it.) On that note: don't fall into the women's-magazine-advice-induced-trap of thinking you can only eat, like, 1200 calories a day. That's a load of crap.
This page http://hospitality.mcmaster. ca/men...nutrition.html
will link you to the nutrition information for quite a few on-campus eateries.
You don't have to count calories to the decimal, but having a general idea of what you're consuming will help you create a balance. So, if you know you need about 2200 calories to maintain your weight, and you grab a chocolate chip muffin as your 3-pm pick-me-up (515 calories), don't go for the Bridges butter chicken at dinner (950 calories).
Oh, and, don't skip breakfast with the hope of saving up calories--you'll just end up starving at noon and making food choices that are less than rational. Also: your body doesn't care what time you eat. You won't get fat if you eat at 1:00am--unless you're consuming an excess of calories.
On the exercise front: it's good to do, but you can maintain your weight without it. But it's still really good to do. For stress-relief, for muscle mass, for bone health, for the ability to have another 400 or so calories to "spend".