to put a connect with facebook for your visitors to login to your website using facebook account you can do is put a connect with facebook button. just follow these steps

1. create a developers account

2. create an app

3. the go to this url: http://developers.facebook.com/docs/plugins/registration/

copy the codes in the examples and be sure you put your appid

then you can retrieve the data with the PHP code sample they give you.