The first step to take is to make sure your teenager's doctor has determined your teen is actually overweight or obese. Some children are simply built differently than others. Their doctor will be able to look at their weight and their stature to figure out whether or not they are truly overweight or obese.
Part of the problem will be your responsibility as a parent. Taking responsibility for what your teen is eating each day is part of your job. If you are not feeding your teen a healthy diet at home, then they're likely eating fast food out with their friends. Moderation is key here. You don't want your teen to feel like they're on a diet all the time. However, by providing a healthy breakfast and dinner, your teen will be able to have a little more leeway when it comes to having lunch with their friends at school.
Speaking of school, do you know what your teen is eating there? Check with the school to find out what food choices are available. If your teen is able to drive, they may be leaving the school grounds each day to buy lunch. Believe it or not, some schools even offer fast food options. Obviously, this is a terrible idea as teens don't know the difference between a healthy meal and junk. They just go with what tastes good, so you must do some educating of your teen when it comes to nutrition.
You should also find out whether your teen really wants to lose weight. You can't do it for them. They have to have a reason for themselves to know why they want to, or need to, lose weight. This comes down to self-confidence and self motivation. Have a serious talk with them about the fact they should not want to lose weight for other people. They should want to lose weight for themselves and their health. You don't want to set them up for an eating disorder in the future simply because they're trying to please other people.
If you child does develop Type 2 diabetes, it is not a condition your child must just live with. By making easy changes to your teen's daily routine, its possible to protect their heart, kidneys, eyes and limbs from the damage often caused by diabetes, and eliminate their risk for developing many of the complications.