Many people want to build a website and sure they want interact with a Facebook, but how to setup their Facebook Application? It’s really not that hard to do, so here's the basics of what you need to do.
You'll need to visit the Apps section in the Facebook Developer Home area. Here you must click on "Create new application".
After the creation you can see the two keys: the App ID and the App secret. Never give out your Application Secret. If you give this out by accident, then use the Reset Secret Key link to get a new one.
If you want to connect some facebook website plugins to your application, like the facebook comments, then you need only your App ID, but if you want to build some php script on your web server to comunicate with facebook, then you need even the App secret.
Anyway, from here, you want to go into the settings. If you are not building an app on facebook, with “Canvas” etc.. , the basic setup is really simple:
Choose a name of your Application to make sense. A good idea is to use the same name as your website or your domain name, because that name is what the users will see, but you can call it whatever you want.
If you need only an interaction with your website and not a canvas application then the App Namespace is not a big deal, but it will be very important the App Domain and the Website Address in the bottom settings.
I also recommend uploading good icons and/or logos for your site.
In the Advanced Settings usually you don't need to make other changes, but I suggest to take a look to the contact settings:
There is a button that will create automatically a Facebook page related to your application, if you do it you can save a lot of time.
Update 2013 video procedure: