http://evilcyber.com/fitness/home-workout-plan-for-teenagers/ this is another great site with the added bonus of training videos and suggestions for training props so you don't have to buy expensive equipment or gym memberships.
http://www.mhp.gov.on.ca/en/active-living/exercise.asp tells you why exercise is vital to your physical and mental wellbeing. Lots of great info from the government of Ontario.
http://www.participaction.com/get-informed/physical-activity-guidelines/ Participaction! Some great info on how to get moving!
http://libguides.uvic.ca/content.php?pid=200993&sid=1679845 Scholarly articles on the many facets of physical activity and the workings of the body, as well as statistics that can help with your research projects.