What Is The Treatment For Knee Pain?

Hi left side of my rt knee hurts especially when I stand up I can hardly walk. In the morning I am able to move much better with less pain. After walking a bit I have to practiacally drag my right leg. I have been resting the leg and using a OTC knee brace. I do not remeber any falling dowen or hitting knee on anything. It just started hurting. What is going on?