Skinny guys gain muscle very slowly usually because their naturally high metabolism that keeps them slim also prevents a lot of muscle growth.
This often leads to skinny guys turning to those that offer miracle fixes such as the supplements industry and various bodybuilding magazines that tout the latest super workout or even worse the steroids peddlers.
You don't need that stuff and most of it will not work anyway.
Anyone can build muscle if they follow the scientific rules based on your particular body type physiology and ignore advice from people who can simply luck out on building muscle.
Skinny bodybuilding is all about the pure mechanics of building muscle adjusted to your metabolism speed.
A quick break down of the process is:

• Workout doing resistance training with heavy weights.
  This creates small tears through your muscle fibers.
  This is the first step that sets the scene for muscle growth.
  
• Eat enough food to give the body the energy and nutrients to repair the muscles you have damaged when lifting.
  As a skinny guy, gaining muscle is often stifled because you do not eat enough food to give you this energy.
  This means you end up working out and the muscle only repair over time back to the size they were.
  Confused? Read on...
  
• Rest and recovery are where muscles grow.
  The workouts you have done and the food you have eaten and turned into energy now get to work when you are at rest and asleep.
  The damaged fibers repair and heal but also get another layer of muscle built over the top a little like scar tissue.
  This extra tissue is more muscle! You have just gained a small amount of extra size and strength.
  
• The more intensity and the better quality of the food you eat will now determine how fast skinny guys gain muscle using the processes above.