Hi, Brittany !
Is your doctor letting you walk around using crutches ? If so, walk on those crutches. Most people don't realize that walking on crutches is not only a great upper body workout, it's a great core and cardio workout too. It takes a lot of effort and strength to be able to walk on crutches. So, use those crutches when you can.
Also, the others are right, just because you can't use your knee does it mean you can't exercise. You can sit in a chair and do chair aerobics. Go to YOUTUBE. search on chair aerobics. you'll find lots of different workouts posted. Even Coach Nicole has a couple of chair workouts posted in the fitness section.
Do you have an resistance bands or hand weights ? If so, use them. Work your upper body muscles while your knee heals.
And the others are quite correct, when it comes to weight loss or even weight gain, what matters most is what we eat. Good nutrition is what takes the weight off and keeps it off. Exercise is what keeps our bodies fit and healthy. In short, you can't outrun a bad diet with exercise. If you want to lose weight, you have to eat right.
BUT !! Here's something very important, DON'T try to diet while your knee is healing. Your body will need those extra calories to help your knee properly heal. If you cut your calories, your body won't have enough calories. So, do your best to eat healthy foods that nourish your body as your knee heals. If you eat crap, your heal will take forever to get better. Try to eat healthy. You'll feel better inside and out if you do.