How can teens lose weight in a week:)

Be smart about it... you certainly don't want to put your body through some ridiculous fad diet.
Cut out, or at least way back, on any junk foods... no sodas, stick with water... exercise everyday... have a healthy breakfast... fill up on healthy foods and snacks (plenty of fruits and veggies)... watch how much you eat, most people eat far more than a single serving size of foods at each meal. You may not lose a large amount, but you should notice an initial loss... then a more gradual change if you stick with it.

Not smart to loose weight fast because you will just put it back on faster than you lost it
Do it the healthy way
Cut 250 calories a day from your food intake
And try to burn 250 calories a day by exercise this equals out to about a pound a week

I find that avoiding late night eating also helps