Here is a post I made on the same subject for another kid trying to get in. It is quoted word for word. It is my advice and opinion. Good luck.
Step 1... put down the fork.
Step 2... push away from the table.
Step 3... Turn off and put away the video games/computer/TV and go for a walk for about an hour. Do 50 pushups a day. They don't have to be all at once. 10 in the morning, afternoon, evening, etc.
Step 4... after walking about an hour everyday for a week, start jogging for 20 minutes everyday, walk the other 40. Increase pushups by 25.
Step 5... After jogging for about 20 minutes everyday for about week, increase by 10 minutes. Increase pushups by 25.
Step 6... Continue the above trend until you are running for 60 minutes EVERYDAY and doing 100 pushups EVERYDAY... maybe take 1 day off... MAYBE.
Feel free to do other cardio besides running like swimming, biking, rowing, stairs but do 30-60 minutes of SOMETHING every freakin day. You don't have to be bored when you work out. But, if you aren't moving 30-60 minutes a day most days of the week, then you aren't serious about weight loss or your health. Remember, you have to burn 3500 excess calories to burn 1 pound of fat... IT AIN'T EASY!
You should do some light weight training 3 times a week too, eventually. You don't have to do that "focus on arms" bullshit either like some people at the gym will tell you to do. You want a whole body work out 3 times a week, unless you want to be a body builder. Then you can "focus on chest" or whatever.
Other ways to get exercise is park as far away from work/school as possible, take the stairs, etc.
As far as food... stop eating fast food. All of it. If you are eating it more than 1-2 times per month, then you are not being serious about weight loss and your health. Eat healthy snacks like fruits and vegetables. There is no limit! Eat em when you are hungry anytime you like! Cut out ALL TRANS FATS, ALL HIGH FRUCTOSE CORN SYRUP, ALL HYDROGENATED (partially, or otherwise) OILS. Those three things alone are responsible for American obesity (besides extreme laziness). SO start reading labels. Eat more fish and white meat like turkey and chicken. NOT PROCESSED food. It has to be real (not mechanically separated, etc) and you have to cook it. Limit red meat to 2/week max. Eat fresh salads a lot. READ THE DRESSING LABEL. 99% of dressings are HORRIBLE for you and have tons of fat. There are some tasty fat free dressings out there now, but still watch out for those three killer ingredients above. Cut out ALL processed/enriched wheat products like white bread, donuts, bagels. Go for whole grain all natural stuff and then limit those to 1-2 servings per day. Nuts also make a good snack in moderation, especially almonds. They have a lot of fat but its all good poly-unsaturated fat. A handful or two will do.
Stop drinking soda and start drinking water... tons of it. At least 16 8 oz glasses per day. It keeps you hydrated and keeps you full.
Thats about it. It seems simple but it is a complete lifestyle change. But there is no secret formula. You simply have to move your body and eat well. No pills, no voodoo, no hypnosis, no fancy gadget is going to help you. You actually have to work. You will be surprised how much better you feel by not drinking soda and eating fast food and can easily drop 10 pounds doing that alone.
Goodluck!
Justin
PS Start taking a multi vitamin. I recommend centrum silver because even though its marketed for older men, it contains stuff to help keep your eyes and prostate healthy and you can never start too early with those things.