How much weight are you trying to lose by Christmas?
Depending on your frame size, you might not really be overweight... maybe all you need to do is tone your muscles and the right kind of exercise can help you do that.
While pizza isn't good for you, I wouldn't go so far as to say you can't have any at all... diets that cut out everything you like are destined to fail... Start out by simply cutting your portions - instead of 3 pieces of pizza, only eat 1 and fill up the rest of the way on salad.
Drink as much water/day as you feel like you need; there's no set amount you have to have, but the more you drink, especially, before and during meals, the less you're likely to eat.
Load up on veggies first, add some lean protein like chicken, fish, lean beef or pork, eggs, dairy, then add some healthy fat such as coconut or olive oil and herbs/spices to flavor your veggies. Stick to whole grains instead of processed foods like white bread, rice, pasta, etc.
Since you are only 15, I hope your parents are already providing the healthy meals you need to maintain healthy weight and nutrition; otherwise, simply cut portions of the food that's provided for you...
It might not be fun. No more pizza nights or pocorn at the movies.Its all about diet and exercise. Drink 8 glasses of water a day. Sleep 8 hours a night. Eat healthy. Don't over eat- eat slowly and don't put much on your plate. Exercise daily, to the point where you are sweating at least an hour. Don't be so stressed. All it takes.