How to login in website using facebook id

Question

I have made a website on which a user has to log in using a username and password. I have seen these in so many website, we can connect through facebook account. And I want

Answer 1

The team at Facebook has spent considerable time documenting this, I would suggest you start there:

http://developers.facebook.com/docs/guides/web#login
http://developers.facebook.com/docs/authentication/