Use RICE:
R=Rest the knee from the painful activity.
I=Ice the affected area for 20 minutes, 3 times a day.
C=Compress the painful area with an elastic bandage.
E=Elevate the leg.
If these don't work, try various pain relievers such as aspirin or ibuprofen (some brand names: Advil, Motrin).
Doctors may also recommend braces or crutches. As a last resort, doctors recommend surgery.
I'm not a doctor, this is not medical advice, but merely information taken from familydoctor.org. I am not responsible for the results of following any of these recommendations.