Don't worry too much about cutting down on how much you eat, unless you are eating a lot of junk.
Concentrate on eating less junky carbs, like white bread, pasta, white rice, etc. Switch to whole wheat bread, whole wheat pasta, and brown rice. Quinoa is a good substitute for white rice too.
Drink a lot of water. I'm up to about 7 liters a day! After I eat a meal, I give myself about 2 hours to polish off a liter bottle.
Another good tip is eat as much protein as you can - it will make you feel more full.
When you go to the gym, try to mix in some weights. Working out with weights burns more calories than walking on a treadmill.
Eat a lot of fiber. I eat Fiber One cereal every morning, along with a whole egg and 2 egg whites.