How To Lose Weight ....for 13 yearolds?

Hi Im Debbies Daughter Amy , And Im Kinda big for my age , I dont EAt Much because im never hungry i drink loads of water ,and i walk home 3 times a week, i cant do much exercising because im scared people will laugh at me :$ but lately iv been putting weight in !!! im not lately i run up and down the stairs for 10 min and so sit ups at night, but i have put some weight on in the past week , but im trying to LOSE it .... Any Hints and fast :D