To create facebook app go to here Facebook Developer
Click on My Apps and select Add a New App.
The popup window open where you can set you app name and email address of your and click on Cretae App Id. New popup comes with enter verification code. Fill correct and your app is created. but you need to setup some settings.
In screenshot you can see you App ready, App id is generated , To view App secret facebook asked you to enter your facebook password. Set your App domain, where you run your APP Like facebook login on your website than add you website domain without http/https and www like example.com
When you creating you app is automatically under development mode only developer can use this app, if you want this app used by every one than make it publicly by click on button in screenshot “Make get page public?”
You app ready to integrate. Thanks