There are 3 forms of exercise: aerobic (running, hiking, swimming, various sports etc.)
Anaerobic (weightlifting, sprinting, etc.)-Short bouts
Flexibility (stretching, flow yoga, etc.)
It is suggested by the Center for Disease Control and Prevention that individuals should engage in 120 minutes of aerobic exercise/week to receive health benefits from exercise. This can include walking, running, hiking, swimming, playing tennis (pretty much whatever activities you like to do). The greatest benefits come from higher intensity (work rate) exercising. (Did you know that one minute of vigorous exercise is equivalent to two minutes of moderate exercise?)
It is suggested by the CDC a MINIMUM of exercising using muscle strengthening activities 2 times per week. In my opinion resistance or strength training needs to be done more often 3-6-days/ week. Girls- don’t be afraid to go in the weight room and have your own routine. Guys might stare, but don't be intimidated. A strong/confident/determined woman is more attractive to guys from what most guys in the gym tell me.
To sum things up you should workout often. You don't have to live in the gym but make sure you are being active! Go on walks or ride your bike to school if you can't make time for a run or the gym! You should do cardio AND weights (weights first then cardio)!
GET STARTED! You'll feel great, and start seeing results in no time!