Using Zapier with Ghost requires that you're using Ghost version 2.19.0 or later.
When you start creating a Ghost Zap, you will be asked to connect your Ghost account.
When you click connect, you will then be asked to enter your Ghost API URL and API Key.
To find those pieces of information you'll want to click the Integration tab:
And then select Zapier. The information will be at the top of the page,
If those steps were correct, you'll get sent back to your Zap where you will now have your Ghost account successfully connected!