Hi nosaj.
Welcome.
You don't say how old you are ? My son who is 27 is like you. He is underweight. He has started eating sensibly and going to a gym, working out. He is getting muscles, and also putting on some weight now. As hanadr says get some calories in you. Cut out any junk foods.
Your BMI if you are 10 stone is 19 ish, that needs to be a bit higher. Don't worry too much. I was also once like you, underweight - now I am trying hard to lose that self same weight. :lol:
Ken.