if you want to gain the health way, eat healthy foods, but in large portions. like instead of eating 1 sandwitch have two. be sure not to do any excersies that burn calories, you can build muscle while you gain though. they say eating alot before you go to bed helps.
to gain muslce, do 50 situps every morning along with a work out plan, and provide your self with enough calouries to come out a little.
To gain weight fast, you need to take heavy meals after every four hours with numerous snacks in between. Consider also taking foods that have high calorie content like nuts, butter, bread and cream cheese. Read http://www.wikihow.com/Gain-Weight.