Following Step by Step to create a Facebook application for your website:
1. Login to Facebook.
Go to https://www.facebook.com/login.php and log in with your Facebook account.
2. Create Developer Account.
You’ll need Facebook developer account to get started. If you don’t have one upgrade your personal Facebook account to a Facebook Developer account now at https://developers.facebook.com. Skip this step, if you already have a developer account.
3. Create new Facebook app.
Choose Apps in the header navigation and select Add a New App.
4. Choose Website Platform.
You should choose Website at this step:
5. Set a Name.
Choose a name for your app and select Create New Facebook App ID. Then choose a category for your app.
6. Put Contact Email, App Domain and Site URL.
You need to add a contact email for each app. Go to dashboard page of your app and select Settings menu in the left sidebar and provide info of Contact Email, App Domains and Site URL:
7. Public your app.
Go to dashboard page of your app and select App Review menu in the left sidebar then public your app.
8. App ID and App Secret.
Finally, you could see App ID and App Secret info of your app.
More info at https://developers.facebook.com/docs/apps/register