To register a Facebook App, make sure you had a Facebook account then get through these steps:
Step 1: Go to https://developers.facebook.com/apps.
Step 2: Go to the Facebook Apps and click the button “Add a New App". Enter your App name and email contact. you will be redirected to Facebook app dashboard page
Step 3: Go to Settings > Basic and fill detail as below. Please double check your App domains (Your Online Store Domain)
Step 4: Click Save Changes.
Step 5: Public your App to Live status by click button at top: OFF > ON
Your Facebook App ID available at top.