Develop a program. Decide which exercises you'll do on which days. Some exercises focus on a specific muscle (e.g. bicep curl) while other exercises call upon several muscles at once (e.g. squat). The important thing is to make sure each muscle gets a chance to rest for at least a full day before using it again. For example, you can do a full-body weight training routine every other day (with cardio in between, if you want) or you can alternate muscle groups (arms on Monday, legs on Tuesday, upper back and chest on Wednesday, etc.).
Muscles grow during rest, not during training. If you don't give a muscle adequate time to recover, you'll actually interfere with the muscle building process. When you lift weight, you're supposed to stress the muscle to the extent that it breaks down at the cellular level, resulting in increased protein synthesis, which produces thicker muscle fibers. This process begins 2-4 hours after the workout and lasts 24 hours[2] (although some researchers suggest that muscles worked to exhaustion need 36-48 hours to rebuild[3]). If you stress the muscle again before the process is complete, you'll break down the muscle fibers before they've had a chance to rebuild.
To build volume rather than strength, design your program around lots of repetitions (10 to 12), three to five sets, and short (30- to 90-second) rest periods between sets. Athletes looking for power and strength, on the other hand, will favor a program with reps that max out at around six and sets ranging from two to six, with a long rest period (two to five minutes) to promote better recovery between sets.
Practice good form. Learn how to do each exercise properly (full extension of movement, proper stance and posture, etc.) or else you'll not only reduce the effectiveness of the movement, but you'll also be increasing the risk of injury. To master each exercise, learn to do it slowly with light weight. Your form will improve with practice. Even though you might feel more accomplished when you move quickly, you're depending on momentum to do some of the work for you, instead of your muscles. Do each rep slowly, in a controlled and steady fashion.[1] More advanced lifters are able to benefit to a significant extent through explosive repetitions, but since this also compounds the risk of injury in novice athletes, it is solely recommended for more advanced athletes.
Lift to fatigue. As mentioned earlier, in order to trigger the muscle building process, you need to stress the muscle. By the final rep of your final set, your muscle should be exhausted.[2] That should be the last rep you can do with perfect form. If you start losing form before then because your muscles are too tired, you're using too much weight. If you find that you can add in one or more reps in good form, you need to add on some weight.
