You're going to need to starting knowing what you eat instead of just guessing if you have any hope of putting on muscle mass.
Here's a little project for you:
Go sign up for a free account @
www.MyFitnessPal.com.
Starting tomorrow, or whenever is convenient... start tracking the calories of everything you eat. Don't eat differently or anything... just eat as you normally would, but log everything. Do this for seven days in a row... and then find the average calories that you consume every day. This will give you an idea of where you're at and will help you to understand where you need to go from there in order to put on some muscle weight.