how does this work?how much?

so i brought this iphone 4 from craiglist and its a at&t and it has the thing where u can put the simcard. And the phone is working. Its working like an ipod you can go anywhere if u have internet but you cant call/text. So i was wondering how does this work how can i connect and how much will it cost and do i have to get a simcard? and sorry my english is not that great this is my 2nd year from france.