1. The stimulus for growthThe stimulus for growth is muscle breakdown.  When muscles are broken down the body sees this as a signal to start building muscle.  Muscle breakdown is typically a result of lifting weights or some other form of strength training.

2. Proper recoveryOnce the muscle has been broken down and the body has been signaled to start building muscle, we need to give the body everything it needs to properly repair the muscle so that it is stronger and bigger. 

Of course, this is a simplified answer for how to get muscles.  There are lots of details that go into each of the two factors.  For example:

-  How much muscle breakdown is appropriate?-  What is the best program to breakdown muscle to elicit growth?-  How many calories and when should I eat to properly repair muscle?-  How many grams of protein, carbs, and fats do I need?-  What supplements accelerate and aid the recovery process?-  What role do hormones play in the muscle building process and how can they be optimized?-  What other recovery methods can I use to increase muscle mass?

My intention is not to leave you with a bunch of unanswered questions, but rather to show you how complex the “simple" process of muscle building really is.  Of course, anyone can get mediocre results without knowing the answers to these questions, but to optimize your muscle gains you have to have the answers to these questions and more.

If you can answer these questions conclusively then you are well on your way to finding the optimal muscle building system for you.  If however, you are not familiar with the science behind muscle building, then I highly recommend seeking-out someone who is.  When you see the gains you get from a scientifically-designed training program you will happy you did.