You might want to look into and get to understand two things called hyperplasia and hypertrophy.
Hypertrophy is the increase in the size of your muscle cells or any other cell for that matter. The bigger your muscle cells are the more water they will hold and the denser they'll look. Supplements like beta alanine and creatine help. Steroids also cause hypertrophy, but the biggest reason I am against steroids is the effects are not permanent. You lose the water as soon as you stop taking them. Testosterone can help with lean muscle gain, but I still don't see it all that beneficial considering the risks and the price. Especially at your age.
Hyperplasia is what you want to happen to increase overall muscle mass. Hyperplasia in bodybuilding is the creation of new muscle fibers and the splitting of muscle cells to create new ones. This is where I and others can throw a lot of bro science your way, but I find what I am about to say true in my experience. You just have to do your own research and make up your own mind:
Human Growth Hormone is abundant in our bodies from the beginning of puberty to the early 20's and is what causes the increase in muscle cells by producing IGF (Google that too). Of course if you aren't lifting heavy at this time you aren't going to grow any new muscle cells which is why skinny guys who weren't athletic as teens and decide to workout in their 30's get frustrated about seeing no muscle gain. Their body is no longer producing HGH and so they aren't able to create new muscle cells. You might still have some time in this window at 20, so get out there, lift heavy and eat, eat, eat. Don't be afraid of carbs. You need them right now. I know you have that hot, lean bod right now, but crabs are important in building muscle.
I do not support the use of HGH either because you can get that gut like Ronnie Coleman. It's nasty, but his physique is an example of what HGH does to your body. I am a big supporter of peptides. I'll give you three links to start, but do your research on these things for sure before you consider them. They aren't a steroid, but still require needles to use.http://www.steroidology.com/forum/anabolic-steroid-forum/603298-igf-1-lr3-vs-des.htmlhttp://www.steroidology.com/forum/peptide-science/628973-peptides-tissue-growth-protection-practical-use-surrounding-them-must-read.htmlhttp://www.ar-r.com/