Im a 16 year old girl, and feel since I've stopped playing netball I've put on like 20kgs, I need a quick and easy way ti loss weight, profer nothing hard on my knees

Swimming and eating less! There is no 'easy' way to lose weight unless you are dropped off on a desert island fending for yourself.

Jogging, swimming and eating healthy. (I got knee problems myself and I walk for about 2-3 hours a day or I do spinning classes at the gym also, I eat healthy and drink lots of water and only water, I cut out all the sodas, sweets and fast foods).
I hope this helps.

Thanks for your adverise