If your doctor has verified that they are colds, it's possible that you are passing the cold back and forth between you and someone else (family member, boy/girl friend, etc), but I'm sure you would notice the other person keeps getting colds, too. The best thing to do is to boost your immune system so you may not get the cold in the first place, but that can take some time to build it up enough.
First and foremost, make sure you are eating a proper diet, getting enough sleep, and doing at least some form of exercise (even if it's walking around) at least a few times a week. That's the basic way to stay healthy in general, but if you find that you are getting sick more often than you would like, there are some natural options to help boost your immune system.
Some of the most popular natural products for boosting the immune system are Colostrum, Mushrooms (Maitake, Shiitake, and Reishi), IP-6 (Inositol Hexaphosphate), Beta Glucans, Elderberry, and any antioxidants, all of which are safe to take in proper doses at your age. Good luck and I hope I helped!
Health/nutrition classes, personal experience, and personal research in my role as a Product Specialist for a vitamin company