Well an overweight young boy, cut down on things like crisps, chocolates, macdonalds, any fast food, fizzy drinks. and start eating more fruits, vegetables, fish
A 12 year old can get some money by selling stuff on ebay, by doing a paper round, washing cars - i dont know be creative

don't diet
thats very unhealthy for a young boy like you
you should eat wise (avoid fatty junks and fast foods)
and exercise...get into cardio...which is infact jogging, cycling, swimming etc.

I can solve both your problems in one single go!
You want to drop some weight? You need to sweat off some calories there is no way around that. You need to earn some money? You need to earn it through working. Summer is just around the corner. Perfect timing it's called YARD WORK, and that's what you'll be doing this summer. Outside, in the sunshine (sure beats sitting indoors watching TV: nobody will pay you for that and anyway you can do it all winter long). You can do anything from painting fences to weeding gardens to mowing laws to pruning hedges, and more. Get somebody to teach you how to prune trees rather than make a mess of it though. Anyway, put up your company advertisement on the bulletin boards of local groceries. Also, you must get a community newspaper delivered to your door find out what it is and they will let you (probably) put in a free advertisment regarding your company services. Im sure your teacher or someone like that can help to get you started as a young and physically fit, suntanned, entrepeneur! Go for it!

Burn more calories than you eat. Put down the game console and ride a bike. Do something besides sit on the couch and watch tv and play games. Mow lawns and clean yards for money.

You really want to get out there and get active with sports. You'll bulk up in muscle and lose the fat.
12 year olds don't generally make a lot of money. If your parents are into it, you could start doing chores around the house. Or you could find people around the neighborhood who have little jobs you could do. Or you could go on eBay and try to think of things that are sold in your area that you can get cheap but other people will pay more for.

Eat a little less cut out the snaks and soda, and be more active. Believe me it works! I hit 280 and in a year I went to under 200!

diet high in fruits and veggies, low in fat, strength training and cardio.

snatch an old lady's purse and run away really fast. you'll get money and lose weight.