Have you tried talking to your parents? I would hope that, as a 13 year old at high risk, they would be open and receptive to working with you to make some changes - if not to the way everyone eats, at least with regards to helping YOU out. I'm a mother of a preschooler with T1 diabetes, and let me tell you, I would do ANYTHING to make my son well. Unfortunately, with T1 changes to diet and lifestyle don't do a darned thing.....but with T2, which is what you're at risk for, you CAN. You can make changes and prevent this disease, and I would encourage you to talk with your parents about how you're feeling. Even if they aren't willing to completely overhaul the way they eat, would they make some allowances? Maybe 1 or 2% milk instead of whole, diet soda instead of regular, not adding butter to the veggies before they hit the table...things like that? Maybe your mom can start buying veggies that you can cut up and keep in the fridge for snacks....buy some popcorn that you can eat instead of chips.....buy some fruit just for you? It doesn't mean they ALL have to change the way they eat (though that wouldn't be a bad thing!), just that they start considering you and your health a little more.
If for any reason that gets you nowhere, then you'll have to do what you can - try to limit your intake of refined carbohydrates (white bread, white rice, processed baked goods..snack cakes, cookies, crackers, chips, etc.) and sugars (regular soda, candy, etc.) and focus on eating lean protein (chicken breast, turkey, fish, etc.) and veggies as the "bulk" of your meals. You do also need a certain amount of healthy fat in your diet - avocado, nuts (just don't overdo the portions!), and olive oil are great sources. When you eat, you can help prevent a spike in blood sugar by balancing your meals with protein AND carbs - so, an apple with a bit of natural peanut butter, for example, or a cheese string with some triscuits. At mealtimes, try to find little ways to control the calories - have a smaller portion of whatever is there, scrape cheese off of things, etc. Also, that dreaded word - exercise. Even just as brisk a walk as you can manage, 20 minutes a few times a week is a great place to start.
I wish you all the best, and really hope that you will be able to talk with your parents and resolve this - changing habits is hard, but I have to believe that they would do whatever it takes to keep you healthy.