Hi! According to Ayurveda, weight below the normal is seen people who have a Vata constitution.
The first thing is to balance the Vata Constitution by getting the correct number of hours of sleep (at least 7-8 hours).

To know your Prakriti, you can download the Vedique Diet App for Android and iOS.

Also, one should consume foods which improve the absorption of nutrients (like ghee, honey)  Taking amla regularly also helps in making the body more balanced. In the winter season start taking chyawanprash. Last but not least, start weight training, it is wonderful to build a toned body.