OK, long answer, skip to the quick summary at the end if you can't be bothered to read it all! Increase lean body mass whilst not overly increasing fat body mass. I.e. increase muscle. When you exercise, you generally do either cardiovascular or anaerobic exercise. CV is often called aerobic, because the metabolism pathways used to provide energy for the muscles is aerobic and involves using oxygen to oxidise food substrates such as carbohydrates and fats. Hence why when you perform this exercise you breath hard at the same time, e.g. running, cycling, rowing etc. This keeps your heart and lungs healthy, and reduces fat, but doesn't have a huge effect on muscle size. The other type of exercise in anaerobic, whereby the body relies on glycogen stores to provide the muscles with enough energy to contract. This is metabolised without using oxygen, i.e. it is anaerobic, and produces lactic acid, which causes the area to ache. When you stop, you begin to breath hard in order to break down the lactic acid buildup and provide the cells with the oxygen they have been depleted of. This sort of exercise is more weights based and is better employed to build up lean body mass e.g. muscle. In reality, most exercises have an element of both, but are predominantly onw or the other. If you can keep it up for more than a few minutes it is probably CV, if you are lifting weights or cannot keep it up for a few minutes and need to rest, it is probably anaerobic. To build up the actual mass, you need to cause microtrauma to the muscles themselves. In essence, to damage them slightly. When you ache the next day or two fater a workout, this is because the muscles are damaged and are repairing themselves. After repeated workouts, the muscles begin to hypertrophy, and grow in size when they repair to better deal with the stresses they are put under. Obviously, they need a supply of nutrients in order to grow, and it is important that you are eating a good source of protein in your diet - meat (leaner preferably such as chicken rather than red meats, as these contains more fats, but either will do), fish, dairy products, and/or vegetarian meat alternatives such as Quorn. Many use protein shakes and supplements which can produce good results quickly and are legal unlike steroids, but can be quite expensive and it is important to make sure that your normal diet is still healthy. I don't use spplements. The muscles also need a good supply of easily accessible energy since muscles use energy even when at rest, so a good supply of carbohydrates are ideal, complex carbs if possible so that the blood sugar levels don't spike. This can include pasta, rice, wholegrain breads and cereals. To produce bigger results faster, it is better focussing on large groups of muscles at once, by using exercises such as the bench press, squat (with weights if possible) and so on, rather than focussing on e.g. bicep curls, which have a lesser effect. It is also important to train opposing muscle groups equally to prevent imbalance which can lead to injury - the body is usually made of opposing musclke groups, so for example you can bend your arms and this is opposed by the muscles that straighten your arms. So in summary - 1) Build muscle mass. To do this perform anaerobic exercises using weights. When the muscles ache and repair, they will grow. Exercise big groups of muscles e.g. squats, bench press for bigger results. 2) Also perform aerobic exercise such as running to keep the heart healthy and not increase body fat. Increasing weight by increasing fat is not ideal. 3) Eat well to provide the body with sufficient nutrients to repair and grow. Include plenty of protein and complex carbohydrates, and water.