If you never created an Application in your Facebok that you might see a Request for permission box, Click Allow on that.
Type your Application Name (example: Ahmed' blog) then select Agree and click on Create Application.
On the next screen you might be asked for a Security Check. Enter the text in the given box and click submit.
On the next screen you will see your application created as shown in screen shot below.
Click on Web Site tab and enter your website URL in Site URL field. Enter your site url with an ending slash (for example: http://mywebsite.com/) in Site URL and click on Save Changes
For Fanbox you will need to copy API Key and paste it in Fanbox settings page in yout website.
After changes are saved click on Back to My Applications.
Next you will see all your Facebook Application listed on left column. By clicking on the one you can see its Application ID, API Key and Application secret.
API key is needed if you are using Fanbox/Likebox. You will also need Page ID for Fanbox/Likebox that you can copy from your Facebook Page URL. Here is a Tutorial for How to create a Facebook Page.
You can manage your created application in Developer Dashboard. Sometimes it takes a few minutes for Facebook to get ready your Application ID to use.