This is a fairly common question. Firstly, there are numerous ways to help your muscles grow fast. That being said, there are some very dangerous techniques that people recommend. It is important to only select methods that will allow you to SAFELY grow your muscle mass. I would recommend discussing with your primary care doctor
or adult doctor) prior to taking any supplements.
To answer your questions, muscles grow in response to use. Therefore, the more work the muscle does, the more it grows. That is why working out (lifting weights) causes an increase in muscle mass. Keep in mind that there are safe and unsafe ways to lift weights. I would recommend working with a physical trainer to learn safe and effective weight lifting techniques! The next step is providing adequate fuel for the muscles to grow, i.e. protein. There are many shakes and powders on the market -- none of which are proven to be better than others. Talk to your trainer about safe methods.
There are many other "medications" that people will say can help. For example "steroids" that you hear about atheletes taking. These are very dangerous and can result in horrible heart problems, shrinking testicles, acne
and kidney failure.
Talk to your doctor for more information.