The simple answer is repetitions and sets. Overloading the muscle causes muscle fibers to break down. Your goal is to reach muscle failure in a safe way while keeping proper form. So you want to use a weight that allows for 12-15 reps for one set. Depending on the muscle bulk you're looking for, you may want to use a little lighter weights and do two sets.

Hypertrophy is the increase in size of a single muscle fiber due to the incorporation of other nuclei from satellite cells to stabilize and strengthen the muscle after trauma. In order to stimulate Hypertrophy it is important to cause this trauma within the muscle. This requires the cells to increase in size and strength to prevent future trauma. Causing the muscle to work under a progressive overload is essential for hypertrophy. The workload placed on the muscle can be increased in two ways. As the muscle adapts to the current workload, the amount of weight can be increased or the number of repetitions or sets can increase. The former is a good option, however, proper form must be the focus because heavy weights tend to cause a loss of form. The latter works until the repetitions reach 15, after that point it is believed that the 'slow-twitch type I' muscle fibers begin to take over. These fibers do not have the same propensity for size gains as the 'fast-twitch type II' fibers.

The best overload system for hypertrophy is a combination of the two increases. Increasing the repetitions with a weight until 12 can be performed in good form and then increasing the weight will cause muscular hypertrophy with a continuous overload on the muscles. The key to growth is increasing the workload as soon as the muscle adapts to the current workout. Keep the muscle under trauma during workouts and allow enough rest time for the body to cause muscle hypertrophy during repair.

Remember that nutrition, food and supplements can also play a role in achieving Hypertrophy. I recommend consulting with a doctor before starting any fitness program, especially an intensive bodybuilding program.