What to do if i want to gain weight to turn it into muscle, but i can't?

Hard gainer: You don't want to get fat first so you can eventually develop bigger muscles. That's a myth. I'm going to assume you are already hitting the gym, including resistance training, hard, and eating a gym diet (lots of protein, avoid greasy & sugary stuff). Chances are that you are a "hard gainer" but most women prefer a lean, defined man who's not massive. Consider creatine & an ems unit if ur dr ok's.