[Matt]

If you take in more calories than you consume, you put on weight. Simple eh?

True. And if you take in less than you burn, you lose weight. But if you're concerned about actually keeping some muscle on, those foods have to be good ones. 1500 calories of fast food is going to be less calories than you burn, but good luck losing weight doing that.

Basically you need to do some cardio minimum 2-3 times a week - any type will do. Also, you obviously need to watch what you're eating. No junk, no simple sugars, nothing high in saturated fat, etc.