Just eat in moderation. Don't overeat and don't eat stressed out over something (ex school work, etc). Try not to eat around others because you can easily over/under eat based on how much others are eating.
Whenever you don't know how much you're eating, stop and listen to your body. Pause at meal stop, and determine if you're still hungry or not. Don't have the mentality that you must clean your plate - but at the same time, don't have the mentality that you must leave food on your plate.
As for portion sizes, here are some good examples:
* For breakfast, if you're having cereal, oatmeal, eggs, or whatever, go with the palm size portion. For cereal, grab about two handfuls, and that will be a good portion size for something like Cheerios. For oatmeal, you'd want to grab about half a hand full (uncooked) or about a handful (cooked), and that will be a good portion size. For eggs, try and grab about a handful.
* Try and drink milk or orange juice. If you can get it in a box, try to get fat free or low fat. Lay off the soda and mixed drinks such as chocolate milk or something like that.
* For chicken, fish, steak, etc follow the card deck rule. Try and eat about the size of a deck of cards. EDIT: This means a deck of card in both height and width. I've seen this guy flipped a piece of steak the other way so that the width matches the size of a deck of card, but the height is like the size of 5 decks of cards.
* Try and eat as much fruits and veggies in each meal. If all they serve are the mixed fruits in syrup, try and dump out the syrup and eat only the fruits. As for veggies, again follow the handful rule. Simply grab about two handfuls of your favorite veggies.
* For bread, and other things, follow the color rule. Try and get whole wheat, and if they don't know, or you can't ask, simply choose brown colored stuff, but remember sometimes white bread is brownish colored. But don't worry, just eat it in moderation and you'll be fine.
* For "stuffers" such as rice cakes, crackers, candies, and so fort, limit it or restrict it. These things will fill you up, and might "fake" you into thinking you're full. Try not to eat things like these.
* Lay off the soda. If you're a soda drinker, limit it or go for diet.
* For cheeses (such as in sandwiches and whatnot) try and remove it from your meal.
I recommend getting a job or finding extra cash so you can go out and buy your own meals.