The first step in your journey to gain weight should be to the doctor's office.
Eat frequently. Don’t let over three to four hours go by without eating.
When you skip meals or go long stretches without eating, you deprive it of the fuel it needs to keep going.
The Mayo Clinic's Katherine Zeratsky, R.D., L.D. suggests a few strategies such as add small, healthful but nutrient dense snacks between meals. "Snack on nuts, peanut butter, cheese, dried fruits and avocados,"
Drinking calories is another great way to add calories, but that doesn't mean you should head for sugar-sweetened beverages, which can create health problems associated with metabolic disorder, including high blood sugar levels and other risk factors for diabetes and heart disease.
Try something healthful and packed with nutrients, like a smoothie of almond milk, berries, bananas and flax seeds.
Go to my journal and copy any of the smoothies that I posted. go to
http://www.medhelp.org/user_journals/show/1040649/Smoothies-and-Protein-Bars
If extra calories won't do the trick alone, consider adding some strength training to bulk up with muscle mass. That will require even more calories.
Make sure that proper nutrition and slow gain are central parts of your plan.