To put it simply, physical activity is good for you! Regular physical activity improves your overall physical health including: reduces your risk of heart disease and diabetes, it increases your bone mass, allows you to live a longer and more fulfilling life, and improves psychological well-being.
The Canadian Society for Exercise Physiology (CSEP) have a list of guidelines of recommended amounts of physical activity for all ages which can be found here.
However it is not just enough to meet the recommended amounts of physical activity to improve your health anymore. It is also important to reduce the amount of time spent sedentary (any activity equal to sitting). CSEP also has guidelines and tips for reducing sedentary behaviors available here.