My knees make a popping sound when I walk what can I do to stop it?

Are you having pain? If it is not painful, then there is nothing to worry about. Most people have some amount of crepitus (which is noise that a joint makes when it moves) in their knees especially. If you have not suffered an injury, if the joint is stable, e.g. not giving out when you walk and if it is painless, there is unlikley to be cause for concern. Maybe mention it to your doctor at your next physical exam.