help!

Whenever I try to make a purchase it won't let me because the person who on the phone before me he keeps going to their Gmail and saying I need to type their password in to confirm it.. for some reason it won't put me on my Gmail I downloaded it for my Gmail account like the Gmail Play store and everything and it still won't work. I don't understand how to fix it I'd like to make a purchase at least try to retrial but I can't because every time my card number and it won't let me do it.