Do you mean you want to put on weight?

Having no fat is a good thing. You should focus on increasing your muscle. 

There are several solutions to increasing your body weight and muscle mass.

You need to maintain a food diary and see how much food you take each day. You can use an app to track the food you eat and how many calories you are consuming. I have used Healthifyme and Myfitnesspal in the past and they have worked great for me. They will tell you exactly how many calories you need to consume to reach your weight goal. Also, make sure you eat a healthy balanced diet and not just anything to put on weight as later it may create problems. The apps will help you track your macro nutrients and you will know if you are eating healthy or not. 

To increase muscle mass you can start exercising, especially focus on bodyweight cardio and strength training. Don't do Aerobics, HIIT, Treadmill, or any such exercises as they do not help to build muscle. Strength training will be the most helpful.

And most important don't focus too much on the number on the weighing scale. It's more important for you to be healthy. Healthy eating and exercise habits will help you get there for sure. Don't try to get overnight results. Focus on forming the right habits.

Hope this helps.

Focus on gaining healthy body weight. Eating protein rich and nutritional food like rice, dry fruits, milk, protein shakes etc. combined with daily exercise can work wonders. Though it will take time!

Most importantly avoid any artificial supplements or medicines, always take doctors guidance before taking any of them.

For increasing protein intake it is best to replace artificial protein powders with a homemade smoothie- blend well 1 cup roasted oats, 1 banana, 5-6 nuts, 2-3 dates and milk/water. This is a great smoothie to increase muscle wait. I have tried it personally.

Why will I want to increase the fat?