Eat more of the foods you can eat. If you don't eat in a healthy matter, you're going to gain fat more then muscle mass. Even drinking ensure, along with meals can help you gain weight. Make sure they are super cold, they taste much better.
One thing I like to use is this stuff called muscle milk. Its unlike any protein drink ive had before, its really easy on the stomach and its loaded with protein but not with sugar which i like about it. It comes in powder or already prepared and lots of flavors. tey have a light version also, thats the one i use. Its by cytosport, kind of expensive but you can find it at GNC. As far as special diets go, I would eat more of the healthy fats liek avacado and nuts, more complex cars like steel cut oatmeal and whole grains like quinoa and brown rice, and lean proteins like chicken and fish like tilapia(also has the helthy fats.) and tons of fruit nd veggies. You need to take in more calories than you put out, so if you eat 500 more calories a day than usual, you will gain a pound a week. Figure out how many caories it is taking to sustain your current weight and about how many calories you naturally burn without excercise and then do the math and fiure out how many calories ou need to be taking in t reach your goal. Also do some strength training. its light excercise without the excessive calorie burn that cardio gives you. Anyways, you can do searches online for the calorie clculators. The more extensive ones are best(the ones that ask you your body frame, heigt weight, level of activity, waist size etc) because they are more specific, and they should gve you a general idea of what you need. I dont have the links but they can be found easily just search for "daily calorie needs calculator" hope that helps you some.