You have a good attitude with this and that's the beginning. The next step will be to start educating yourself. I was about your age when I started training. Go to a bookstore and look for some books on beginning weight training. Start reading magazines like muscle & fitness and flex. When reading the magazines, ignore the supplement info. Over 90% of it is bullshit because most supplements just don't work. Even though all the ads are crap, the training info and dietary advice are pretty sound and will give you a place to start learning. Like the others said focus on your diet. Here's a little sample plan.
Start learning about protien, carbs and fat grams and how much of each different foods contain. While trying to lose fat, keep the carbs (sugars) low. A simple way to keep your diet in check is by eliminating all processed foods. Processed foods are foods that do not occur in nature (all junk food, lunch meats, sodas, chips, etc.) If you are eating naturally occuring foods only, you will be on to a good start. This takes getting used to, so don't try to eliminate all processed foods at once, just eliminate them slowly over 4-6 weeks. Keep protein as the staple of your diet (baked chicken, tuna, lean beef, turkey)
As for the training, just do it whenever you have time. The most important thing is just staying active. I doubt that at this time you are interested in developing a bodybuilder's physique, so your excercise choice should just be something you enjoy doing. Go swimming, bike riding, play tennis, basketball, football, whatever. Just try to do something everyday for an hour or two that gets your heart rate up.
There's a lot to learn, my young friend. Changing the body into what you want it to be is a science. Just start educating yourself and you'll be on your way.
If you have any questions you can pm me, or feel free to email me. I'll help you out however I can.