1) Start drinking more water: When you get up in the morning, drink a bottle of water before you allow yourself to eat anything. Drink another bottle before lunch, and another before dinner. When you get hungary for a snack, make yourself take 5 gulps of water before you can have one. Soda, milk, etc... do not count as "water", but there are some great flavored waters out there, or you can just buy a lemon and cut it into like 10 wedges. Then squeze a wedge into your bottle so it doesn't taste bad. If you don't have money to buy bottled water, get one bottle, and refill it all summer with tap water and your lemon wedges.
2) Remember the mottos "no pain, no gain", and "pain is weakness leaving the body", and "go hard or go home", and "pain is good, its your body's way of telling you that your not dead yet".
3) Begin exercizing. If you are out of shape now, start by light jogging. Don't be afraid to get out of breath. If you have never been a runner before, start right now with going 4 blocks: you run the first block, walk the second block, and run the third block, then walk it home. Go every day, and add one block each day. Once you get up to 10 blocks, start only letting yourself jog slower during the parts you were walking. Cut walking out completely. So then its Run a block as fast as you can, the slowly jog the next block to catch your breath, then run again, etc... When you build this up to about 20 blocks, you will be able to say that you can run 2 miles!!!
4) Don't forget to stretch out each day before and after you run. Look up simple stretches on the internet. Hold each stretch for at least 10 seconds. Learn to love stretching and working out because you love your body and it feels good.
5) The other posters are right, don't concentrate on your weight, concentrate instead on how you look in the mirror, how much energy you have, and how you feel. It is healthy to loose about 2 lbs. per week. But remember, that if you are only 12, you are probably developing and gaining a little weight. This is not all bad. When boys mature, they like girls with curvy hips and breasts. Remember that. Don't let yourself purposely be a stick girl! Be healthy!
Experience in the US Military.