Eat more + weight training.

1. Eat more fat + protein food. You can work on burning the fat later.
2. Start lifting light weights and increase slowly.
3. Get enough sleep everyday

I was at 43kg after graduating in 2012. 23 years old btw. In 2013, something opened my eyes and I decided to change the way I looked. Started eating well...oh and I'm a vegetarian. I also stated going to the gym regularly.
I am now at 59kg which is good for my height (161cm)