Damaged a ligament in my knee. Any advice?

You need to be seen by an orthopedic doctor. If your ligament is actually torn instead of just stretched, you may need a surgery to repair it. If it is just stretched, the doctor will tell you to take it easy for awhile, and send you to a physical therapist to learn some exercises to strengthen the muscles around the ligament, and to hopefully return it back to normal. One good exercise for ligaments in the knee is pushing a car that is in neutral. Also, avoid any cutting sports, like basketball or racquetball until you are healed.