The National Institutes of Health’s (NIH) website provides a wealth of information not only about obesity and diet, but also about health issues in general. The NIH is part of the U.S. Department of Health and Human Services, and is the nation’s medical research agency, the largest source of funding for medical research in the world.
The NIH website has a whole database of information and, if applicable, examples of exercises and instructions on how to perform a physical activity. Walking, for instance, has information on the benefits of walking, how to start a walking program, goals and samples of stretches to complete as a warm-up before walking.
There’s much more available. Check out health.nih.gov for more.