How do you know how many calories you should be eating each day? If you Google BMR (Basal Metabolic Rate) Calculator and put your info in, it will spit out how many calories you can consume per day. That is how many calories you burn if you didn't move all day long. Now, if you worked out and burned 500 calories, you can add that amount to what you got from the BMR calculator. If you eat more calories than your BMR+your workout, you will gain weight. If you consume fewer calories, you will lose weight.
If there is one thing I have learned from watching The Biggest Loser, it is that a pound equals 3500 calories. So, if in one week, your total calorie intake is 3500 calories lower than your weekly caloric allowance, you will lose a pound. If you eat 3500 calories more than your weekly caloric allowance, you will gain a pound. Make sense?
It is all a game. You can pretty much eat whatever you want. If you want to eat that Cold Stone Milkshake in a Love It size for 1427 calories, you are going to need to find a Cold Stone Creamery about 4.5 miles from your house and run there and back at about 6 mph. :)