Make sure you are logged into the gmail account you wish to associate the API key to.
Click on the get started button
Select all 3 checkboxes (unless you are absolutely sure you do not need any of the others) and click CONTINUE
When prompted for a project , either select a current project or create a new one.
You will need to setup your billing account next or associate to an existing account.
Once linked you will receive a popup with your API key , copy it and send it to your developer or use it in your theme or plugin.