What should you do if you have a MCL tear of the knee?

I've had so much trouble with my knees in the past. So I can guarantee I have a MCL tear of the knee. I have to lateral meniscus. I have osteoarthritis in my knee, and well have a plain messed up knee. I injured myself again yesterday, and now I can't walk on my knee, and It hurts to touch the inside of my knee, and it is really swollen. What should I do?