I don't know what to do with my stretch marks. Is there a way to prevent getting more?

Leave it alone. You can't do anything for your already developed stretch marks. Don't get pregnancy, don't grow bigger: but it's unrealistic. Strech marks simply are caused by your skin being streched during pregnancy and growing, perticularly during teenage.
Laser. Stretch marks can be improved but not eliminated. Fractionated laser with erbium or co2 can improve stretch marks in about 3 sessions by rebuilding collagen. Another treatment which has no down time is carboxyhtherapy, injections of carbon dioxide gas, into the skin. The treatment is more prevalent in europe but is available in us. It can improve stretch marks over 50 per cent.
90% women get them. 90% of pregnant women get them to some degree as the baby grows faster than the skin can accommodate. To keep at a minimum don`t gain over a healthy 25-30 pounds; keep hydrated; use hydrating creams and lotions. When pregnancy is over you can see a Dermatologist who can recommend several possible treatments. Don`t stress out-it`s normal.