Any advice loosing weight fast?

im 13, when i was 11 i dislocated me knee a couple moths later after i was told it was heeled it happened again an ddt just kept happening again finally got surgeryey when i was 12 the surgeryey was soposese to be Graniteie but of corse it happens again now almost 2 years later from being on cruches and in wheelchairs sitting around not doing nothing i gained a ton of weight right now i am 5'7 and 222lbs ;[ when i was 11 i was 5'2 and 130lbs i was never stick skinny but now i feel horrible its almost 100lbs more then i was, thats 50 pounds a year ;[ please i need some real advice..my family cant afford for personal trainers on mebership to the gym but i can do things at home like riding my bike wich has been what i been doing also my knee is still no heeled i may need surgrey again but my doctor did tell me if i loose weight it would take stress of my knee and could help it heel so once again please help i really need to loose this weight.