What are good exersizes that can make you lose weight fast? -

in like three weeks lets say, like 20 lbs, im 15 yrs old by the way

You shouldn t really be trying to lose weight at that age if you re not overweight. The best kinds of exercises are the ones that get your heart pumping (cardiovascular), like running, jumping (jump rope, jumping jacks), even speed-walking. These are all things you can do almost anywhere; the only thing is doing them for a long enough time. Your heart should be beating at an increased rate for more than 20 minutes a session. Doing about a total of 4-6 hours a week (not all at once) is reasonable.