Summarize this article:

Consider putting together a basic routine to boost your fitness level. Whether or not you have access to a gym, start with the basics at home: push-ups, pull-ups, sit-ups and, if you have access to weights, basic squats, curls, and bench presses.  If you’re not sure of where to begin—what exercises to include in your routine, what weights to lift, etc.—talk to your gym coach or check online for teen weight-training recommendations. Before each workout, start with about 10 minutes of mild stretches. Warming up helps prevent injury and can help to maximize the calories burnt during your entire workout.  Similarly, if you take the time to taper off your workout with cooling-down stretches you will feel less taxed when you are done. Over time, if you find that you enjoy running, think about joining a cross country team. Find a friend who also wants to be active during the week and exercise with them. You can also ask members of your family (siblings, cousins, even parents) if they’d like to exercise with you. It’ll be easier to stay motivated to be fit if you have a friend or family member to exercise with and who can provide moral support. Having a friend to support you will make you more likely to stick with your exercise regime.

Summary:
Start an at-home exercise routine. Warm up and cool down before and after your workout. Exercise with a friend.