Drink lots of water, get lots of calcium, exercise and watch your portions. You are still a growing girl so don't go on a crash diet (you sound as though you know better but just wanted to mention it).
Watching your portions is the best way to "count calories". Weight Watchers is a great example of this. Yeah, you won't lose 50 lbs in two weeks but you will lose it in a way that is healthy. Also include plenty of calcium in your diet. Yogurt, milk, cottage cheese....low fat options of course. Calcium has been shown to increase weight loss and as a young woman, you need that extra calcium anyway.
Also, exercise exercise exercise. Walking is great. Yoga, pilates, step aerobics....whatever your fancy. Just get moving.
And #1, talk to your doctor about your concerns. You are still a young woman and there may be some concerns about dieting. Your doctor can recommend a good nutritionist for you and give you the go ahead on an exercise program.
Good luck and congratulations on wanting to be a healthier you!