Maggie's Farm

It's not that simple. Exercise WILL help some people lose weight. But many overweight people are genetically programmed to be overweight. There literally are people who no matter how hard they try they cannot lose sufficient weight.
The there is the question of how much and what kind of exercise you do. Run 30 miles a week while lifting weights an hour each day. It's damned hard to become obese doing this. Or walk the PCT from Mexico to Canada averaging 18+ miles a day and eat whatever you want and can carry. You will lose weight.
The best way to maintain a healthy weight is weigh yourself every morning and then use this to decide what you eat that day. AND never get overweight in the first place. It is easier to maintain your weight than to lose a lot of weight. Start in your teens and attain and maintain a weight appropriate to your body/build.
The BMI is an interesting measurement but not handed down from god. If you are a body builder the BMI will rate you obese. If you are a distance runner it will rate you unhealthily underweight.
Put on a speedo (or if you are female a teensy weensy bikini) and stand in front of a full length mirror. If your response is laughing or crying you are not the right weight for your body/build.