New Manager needs help in dealing with a staff member?

I'm in a bit of a situation I got promoted to a managerial position but as this will be my first managers position I have no training whatsoever. My cashier however has studied Management and thinks he is in charge everything I tell him to do or not he does the exact opposite. I have only been in this position 3 days Every time I tell him something he doesn't reply yes/ ok or something to acknowledge that he understands. He undermines me in a way but not terribly well yet anyway. I don't want to get mad at him but I feel like I have to repeat myself again and again. I don't think he takes me seriously. I need suggestion on how to deal with this in a professional way?