New workout needed fr weightloss

I am 40 pounds overweight and 19 years old. I don't have any weights and I can't run because I have bad knees. I need a flat stomach, toned arms and to lose weight in the thighs and upper leg area. I don't know what exersices to do....so far I run up and down 6 flights of stairs which I'm not sure if thats okay for my knees (they're loose), do 60 crunches in 1 sitting, walk for an hour, and as many push ups as I can manage.