How to login through facebook and if user not registered in my website then first register him and then rediterct him to website's page.

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Facebook-API

ASP.NET4

I am using login with facebook button with registraion in my website. So I want to know how to implement the following:
1. When user clicks on "Login with Facebook" then it should check if the user previously registered in my website or not.
2. If user previously registered in my website then he simply redirects to my website's home page.
3. If user has not registered in my website then he redirect to the registration page, where I am using "Facebook Registration Plugin".
4. Then using "Facebook Registration Plugin" he will register in my website.

Overall I want to ask how the Sign-In with facebook of log in to codeproject.com with Facebook implemented.