I am very thin . how to become fat .my height is 172 cm and weight fifty . age twenty . my penis also thin .

What do you want?: You are a young man and you almost certainly want lean muscle, not fat. You are probably also light-framed like brad pitt -- it's a plus overall. Hit the gym and do resistance work. Make friends with guys who lift. Ask your physician if creatine and/or an electronic muscle stimulator is in order. Plenty of protein, consider a cheap protein supplement. Your penis won't change & is probably fine.