Eat fruits, vegetables, and grains. Stay way from thing such as fast food, while they don't weaken your immune system, they don't help either. Trying to be less stressed can help as well. Exercising to a certain limit, can be very helpful and beneficial in more ways than one.
You need to maintain a healthy weight, exercise at least 30 minutes or more a day, washing your hand thoroughly and often with soap and warm water, try to get at least 8 hours of sleep everyday (depends on your age as a teen) , eat food that contains vitamin c (eat healthy and right) and don't do drugs in order to strengthen your immune system.
Tell to you the truth, the only times I get sick is when I really barley get any sleep (5 hours straight for a week) or allergies. And I really just eat the same daily. Cerial, food, drink water(but not always) and exercise helps too. But I really don't do it either, I guess I was just born with a great immune system.