Do you think you eat too much? Or do you think you need more exercise? Well for most people, they seem to think over eating is the biggest problem.
But that might not be entirely true.
One article from the LA Times points to lack of exercise. I personally think it's a combination, and everyone is different.
The best way to find out is to track both your exercise and eating for 2 weeks. Then take those data to a personal trainer and have him or her go over it with you. Then come up with a plan of action.