Find your Ad Account ID in your Business Manager 

1. Go to your Business Manager account > Ads Manager

2. In Ads Manager, click Facebook Ad Account dropdown menu, and you'll see numbers after Account #: xxxxxxxxxxxxx. Choose the ad account ID from the ad account that you'd like to use for your Facebook campaigns! 

If you do not have a Business Manager account, find your Ad Account ID in your personal account. 

1. Go to your personal account and click on the dropdown menu on the top right.

2. Click on Create Ads or Manage Ads, it should direct you to your personal ad account!