You might be skinny because of lack of sufficient foods or apparently the foods that you are consuming are lacking enough nutrients.
Gaining weight is something that you shouldn't worry about.
Here are the simple ways you need to know as quoted on stronglifts.com
1. Lift heavy weights
This is a form of exercise as well as building your body. You should do this on a daily basis in order to achieve your aspirations. The more weight you lift, the more muscles your body will develop.
You can also resort to doing exercises like press ups, jogging, among others.
2. Eat more protein
Proteins are body building foods and thus it will be a good idea if you consider food laced with protein. Among the foods include meat, beans, eggs and fish.
3. Generally, eat more food
As mentioned earlier, you might be underweight due to lack of enough food. Eat like a horse, consume more calories. Advantage of eating a lot food is that the body will get and absorb more nutrients that will in turn cause weight gain.
4. Go liquids
You are also recommended to consider blended foods because they are easy to digest. Here is one example of blended food, a mixture of milk, banana, peanut butter and whey protein.