How can a 17 yr old lose weight?

im 17 and tryin to get in shape for track next year, the past 6 months i havent been able to do anything due to surgery on my knee cap. I have gained 20 pounds and its ****** up, i used to be in such good shape and im al chubby now iv been sitin around the house eatin terrible foods for months. i cant go straight to the gym to burn it off sense im steal healing. how can i mak a diet for myself to lose 20 pounds in 2 months or less. i cant stick with anything so if i just have the diet set up for me i wont stop doing it. Thanks