Follow to simple steps to acquire the API key:
1.Login into your Facebook account.
2.Then go to Facebook Developer application and click on the Apps Link
3.Click on Create New App and new window will come as following and enter the App name and other details and click on Create App
4.Then go to the Apps Page by clicking here or click on App Option on the top navigation bar,then choose the app you have created and you can see the API key and the App secret key below the Settings Label.
Now that you know how to acquire the Facebook API key and secret number, you can start using facebook connect and other developer tools.