The shortest answer...resistance training. Easiest way...lift weight. Now for details.
Not all body types are the same and certainly genetics play a HUGE role in how your body will respond to the exercises. However, that doesn't mean that you can or can't build muscle. It only means the way you train may have to vary.
If you are genetically predisposed to building muscle such as most of your professional athletes like body builders, football defensive and offensive linemen, wrestlers, etc., the muscle will come fairly quickly but for others...not so much.
For the average Jo, such as myself, I need specific training to gain muscle and my nutrition plays a role as well. My hubpages offer more information but to start, give this plan a try.
Perform this routine 3 days a week for four weeks. Give a days rest in between exercise and use proper form and technique. Do a warm-up set of 10 at 50 to 60% of max weight. Then do 3 sets of 8 reps at 85 to 90 % of max working to failure on the last set. (typically failure would be reached on rep 6 to 8.) Rest a max of 30 to 45 seconds between each rep.
clean and jerk
bent over row
Again, my hub Build Muscle Bulk has much more information if you are interested. Good luck in your training.