How Much Should I Exorcise In A Week?

Cause I don't exorcise at all. I'm afraid it'll either make me lose weight or gain muscle. Muscle to me is very unattractive and I like the bony look. Also I don't need to lose weight cause I'm already a little underweight. Is it neccasary to exorcise to be healthy and if so how much should I exorcise if I don't want to gain muscle or lose weight?