I have no insurance or money to see a doctor. What can I do to treat bad joints?

Lose weight and walk. The greatest evil to joints is excess weight. Losing weight requires discipline, education regarding food choices, and exercise also can be helpful in the process. You can do all of this without seeing a doctor or spending any additional money on food. All that being said, seeing a physician for advice and a physical exam can cost some money up front, but can save you much more in the long run.
Exercise. Interestingly, many cases of "joint pain" have nothing to do with the joints themselves, but more to do with the muscles, ligaments and structures around the joint. Strengthening these supporting muscles will tighten and stabilize the joints, and reduce wear by maintaining normal function. A great treatment for knee pain is stationary bicycling. Important tip, though: use the toe clips!