Hi everyone ! I am 19years old and I have a slim body from which I am too much fed up.
I want to put up some weight just to get a athletic body. So, suggest which kind of diet I should take and should I join the jim also for it ?...

You need to give your height and body weight so that one can determine your current body structure. Though you need to eat a lot along with moderate exercise in GYM. Later on, shift to weight training session after one month. Continue weight training for atleast 3 months with constant effort to increase strength.