How can a 12 year old lose weight in 1 week?

You are a bit young to worry about your weight, but...

After you eat go for a walk to boost your metabolism which burns calories faster. Before you go to bed maybe do a couple crunches. Eat healthy and if you starve yourself I will go up to you and snap you in half :D eat lots of fruits and vegetables and cut down (cut down; NOT cut Out) on Carbs.

Remember: you will slim up once you hit puberty. :D