I am really underweight and recently its become a serious concern. i am 5'8" and i weigh 45 kg's. i am considering joining a gym. should i?

See below: Welcome if you are underweight your bones maybe a little weak exercise helps build up bone also increase your caloric intake t gain some weight protein is good to build up muscle.

Absolutely: It sounds like a fantastic idea. I'm less concerned about your weight than that you have to ask. You've just recently become a grown man and this is what guys who will practice lifetime fitness will do. Eat plenty of protein but avoid extra calories in greasy and sugary stuff. You'll make friends doing fitness activities and they'll guide you as you develop the lifestyle & body you'll love.