I am under the impression that to use an unlocked phone, I just slide in my sim card and start using the phone. BUT this is my first smartphone so I do not have a simcard. Does this mean I have to first buy a smartphone and get my prepaid plan with AT&T in order to get a account and simcard, then switch to my unlocked phone by sliding in my simcard that way? OR Is it possible I just buy a prepaid simcard for AT&T, and use it on my unlocked phone that way?