Okay so when your behind a girl whos "dancing" aka grinding on you what do you do?

okay so many times i find myself at the club dancing with a girl (im a guy btw just in case your wondering) okay so im dancing with a a girl and then she turns around and shakes her *** and grinds on me..dont get me wrong i like it lol but im not sure ABOUT my role in this...what i do is put my hands on her hips and move to her rythym but what do i do when she starts poppin her ?? do i just stay there os should i move my waist front to back or what?? and girls do you lguys like it if your grindin ona guy and you feel something hard?? i try not to get hard cuz i dont wnat girls thinkin im a perv?? okay well any ti[s will be appreciated...btw im 20 and recently started goin clubbin so i dont have the most experience at doin these things and in high school i was a pot head and didnt really go to the dances so i dont have any previous experience at this sorta thing