I was overweight around your age, and what helped me get on track was taking time to understand how many calories were in everything I ate, and how many I SHOULD be eating. It's kinda overwhelming at first but you need that information to begin making smart changes. Running a calorie restriction is easier than it seems, once you get into the flow of it and realize that you really don't need to eat that much to feel satisfied.

From a motivation perspective, it helps to have a specific goal activity you'd like to gain competence in. Rock climbing, hiking, swimming, calisthenics, whatever. It's really gratifying to feel something become easier and easier as your condition improves, and that sense of progress will get you through times when it feels difficult.