The first thing you need for a Google Maps API key is a Google account with billing enabled. To ensure that you do, visit the Google Maps Platform and click on one of the blue Get Started buttons.

Next, click Google Cloud Platform home in the top-left corner. Then click Billing to ensure that you have a payment method on file.

After confirming that your Google account's billing is active, click the top-left Google Cloud Platform link again. Next, hover the cursor over APIs & Services, then navigate to Credentials. If you have an existing project you'd like to use, choose it from the list. Click on "Create a new project" and give it a name if you do not have an existing project.

Create credentials (pick API key). A new dialog will appear and show you your new API key.

Next, close the API key dialogue. Your API key will now be listed on the Credentials page under API keys.