What to do for a girl that is mad at you?

my ex girlfriend found out i told someone me and her had before even going out for a week and now she said she NEVER wants to talk to me again. i really do still like her and me and her were prolly going to get back together bc she still liked me.. but now she hates me. i was going to go to prom with her too. i was thinking about leaving like roses and a sorry card on her doorstep or something that will make her forgive me? please give me some idea on what to do i really need her back =[