The MCL keeps your knee from moving sideways. It can be damaged by your knee being pushed inwards, and sports are the usual reason for it being damaged.
Icing your knee is important. Using crushed ice wrapped in a towel, put it directly on the injury for 20 minutes. Wait 1 hour between times when you ice it.
Your doctor may have you wear a knee brace to protect your knee, and you'll need to make sure that it doesn't have any sideways motion like the motion that caused the injury. You may have crutches to use so you don't put your weight on your knee while it heals.
Physical therapy can help you strengthen the muscles around your knee to protect it and help you regain your mobility.