What is a good way to get exercise after tearing your knee cartilage?

Low impact exercises. Low impact exercises are generally a good idea when one is suffering from knee injuries. Riding a stationary bike, elliptical machines, or swimming are exercises that will help strengthen the muscles around the knee without causing significant discomfort. Avoid squats or lunges.
Depends. Assuming that you mean a meniscus tear, usually low-impact exercise such as a stationary bike or eliptical machine can help improve your range-of-motion and strength.