First, you have to sign up for the Google API services and then go to the Credentials Page and click on the Create Project.
Provide the Name as Real Estate Manager and click on Create.
Now you need to create an API key for this project. Click on the Create Credentials -> API Key
Now you have to click on the RESTRICT KEY
and then choose HTTP referrers (web sites) and add your website url.
The most important thing here is to add the homepage URL and then a slash and an Asterisk (*).
The purpose of this is to allow the API calls not only from the homepage but also all the other property pages.