I know exactly how you feel. I am about to turn 15 this december and I have had the same problem for about 6 years now. When I was about 8 years old I weighed a good 140 pounds. My doctor was concerned about my weight but never told me how to lose it. So my mom was feeding me veggies and fruits and I played soccer as a sport. But my weight still seemed to increase. As I reached the age of 10 I had dropped about 20 pounds but found later that year that I had gained back 10 only to find that it was muscle( So even though your body weight may be a little higher than normal, you could just be very well muscle built) And now that I am 14 5'5 and 163 pounds. Im still trying to drop but am still having problems. So I tried to start off little.
Usually what I do is watch t.v. shows and see some simple exercise methods. Such as doing sit ups (about 10 -20 each day) or just dancing to music.
If I ever get hungy Near a meal time I will try to have a light snack such as a special k bar or some fruit. When meal time comes around (I oviously dont tell my mom Im trying to lose a few pounds because she becomes quite furious) but I take smaller proportions.
And about the doctor, I know exactly, I mean exactly how you feel. Every time I go for a checkup she always talks about me losing weight. She says the average weight for a 13-14 year old is about 140-150 pounds.
I knows its hard but just try to keep a positive attitude, try some simple exersice methods and before you know it your on your way to losing weight.
P.s. I heard on the radio one time about a girl who was 11 and weighed about 200 pounds. The doctor told her a really rediculous eating habit that I actually tried and found worked. If you tend to eat your food quite quickly and swallow your body doesnt get as full as fast, so it tends to make you even hungrier. But if you take the time to chew your food ( try about 10 times per bite or longer) and eat slowly then you wont feel as hungy. I know its sound rediculous but sooner or later you find yourself doing it automatically.
Hope I helped