please i cant do anything with my phone

I have the iPhone 4s I bought it a week ago on craigslist from a girl for $150 when I got home and charged it the screen said hello and wanted me to pick the language and wifi network after I did it on top it said iPhone locked and below said enter apple id and password and wont let me do anything else at all so I tried to call the girl and she wont answer so please if anyone knows what I can do let me know I really need it because I don't have money to buy another phone and don't have one to use while I'm waiting for this