What Causes Pain In The Hip , Knee And Legs?

Yes, I am in constant pain in my hips,knees and legs. If I sit a long time and get ready to get up my legs are stiff and makes it hard for me to walk. I have constant aches at night at well. I have been told I have osteoarthritis in the past. What can I do to relieve the pain and what kind of doctor should I see.