about gaining muscle...?

After puberty, and especially after adolescents, it's actually much easier to build muscles because your metabolism begins to become lower.

I used to have a hard time bulking up. I kinda looked like a shrimp no matter what I did and how much time I spent in the gym.

I used all kinds of protein powders and stuff like that but nothing seemed to work, until a guy that worked out with me pointed me to this program that I now have been using for a little less than a year. It taught me all the fundamentals of muscle building and eating right to fuel my muscles so now I’ve gained some pounds and my body is starting to look the way I want.

Good luck :)

Wrong. After puberty, your muscles will stop developing by themselves, but you still can induce further growth through training. I'm nearly the same age as you, I do weights 4 days a week and I'm putting on muscles fine. In fact, bodybuilders generally don't reach their "peak" till around the age of 35, I hear. 20 is a pretty good age to start because you're still young and your testosterone level is still fairly high.

This is simple. All of your questions can be answered at bodybuilding.com.

WTF! Ofcourse you can.Eat high calorie food (based on your body type).I recommend to buy a good protein supplement too.