If you must sit at school and/or work, then move your legs and body about, but not constantly like a pendulum. Get up and go to the pencil sharpener, if possible. Step outside during your lunch break or walk to get a drink of water.
Aerobic exercises are those that get your heart-rate up the most. They will improve your body’s ability to handle physical exertion and make you healthier.
Be consistent. If you want to get in shape, you will need to get in some activity at least every other day. You can’t expect results when you exercise inconsistently and infrequently. I have a plan easy to follow.
Sugar, salt, and unhealthy fats will contribute to keeping you from getting in shape. Avoid sugary drinks like soda, and anything which is high in saturated or trans fats (partially hardened or hardened fats, margarine). Instead, eat fruit as a dessert and foods with healthy fats like omega-3’s (found most easily in fish, olive oil and nuts).