PLEASE HELP ME OMG PLEASE?

ok so i just traded my old phone for a sidekick lx with this guy about 2 days ago. they cancelled their connection so i had to buy a new sim card. i paid 30 dollars for it and the guy at the t mobile store helped me out. however im at home and i cant sign on aim or use internet bc it says i have to "sign In" so i went to sign in from the menu and it shows sum1s screename for the sidekick and its asking for a pword. this is not for aim im talking about the sidekick sign in screename and pword. plz help me. if u can tell me exactly wat to do plz! then wen i try to power off and then turn it bak on, it sayd i can register but wen i do tha it says the sim is registered already. basically he gave me the phone with his old sim card in it. i took it out and the guy at the store activated the new 1 i jus bought and its in the phone now but its asking me to sign in and i dont even no the sn. its showing me the guys old sn plz heklp me