Since your mother buys the food, you'll have to enlist her help in your weight loss efforts. Ask her to make sure there are plenty of fresh veggies for every meal, as well fresh raw veggies to snack on, fruit, and plenty of lean protein.
You'll have to give up things made with white flour and sugar, such as breads, pastries, sodas, etc -- simple carbs.
Exercise is very important and there's almost always time to do a little bit.
Try walking to school, participate in sports, etc. Even 10 minutes of exercise is better than none at all.
Your only valid excuse is that you are not buying your food. There is always time for exercise. Keep a food journal, keep track of calories, stop eating breads etc (carbs), no eating after 7pm and drink water all day, oh and stop with the excuses. These steps alone will make u lose weight. I went from 200 to 120lbs with no exercise. My results wiuld have been so much faster if i would have just jogged