Some foods fill you up more quickly than others. If you start with the wrong foods, like pasta or vegetables, you'll still have plenty of room left for everything else on your plate. That way, you'll eat lots more than you planned to.
Instead, start with more filling foods. Then you'll cut down on the other calorie sources.
The best way to start is with a bowl of soup. Then have some beef or chicken. Maybe you won't have room for all of your vegetables, but you'll consume fewer calories in the long run.