Most people who say that they eat alot but don't gain weight, are simply not eating enough. You may think you are eating enough, when in fact when you are not. I can speak from experience, I'm similar to you, 5'8, 20yrs old and I was 103.4lbs in July, but I am at 123.2lbs now.
Anyways, this article should get you going.
http://www.discussbodybuilding.com/m_111173/mpage_1/tm.htm