You need not generate your hash key. When you will try to access facebook from your app, you will get an error msg saying following "[Hash Key]" is not registered or configured. Just copy that key and go to developer.facebook.com where you have registered your app for facebook verification and include under key hashes.
Remember : For every device, the hash key may be different. You need to include it freshly.
By default this key should do : 2jmj7l5rSw0yVb/vlWAYkK/YBwk=
Also, before everything, you need to provide facebook with your App details, along with screenshots of where you are using facebook features and reasons for why you are using. They need to verify that.