Just like someone trying to lose weight you need to monitor your calories. Except you need to be at a significant calorie surplus, and hit the weight room with vigor.
At 100 pounds you would probably do well gaining weight at around 3000 or 2500 calories. You can't just eat and think you are eating a lot and expect to gain weight. You need to document everything.
And not any old food will do. Healthy food, fruits veggies, whole grains, lean meats. You cannot eat junk food and expect to put on quality lean mass.
Its going to take a lot of diligence in the kitchen and with your nutrition to be gaining weight.
It basically boils down to this: heavy lifting, calorie surplus, consistency.
A good program to start with is: StrongLifts 5x5: The Ultimate Strength and Muscle Program | StrongLifts.com
Good luck and don't give up early, it takes lots of time. You will gain about .5-1 pound per week, if you stick with the program. Over a few months that will really add up.