Click on Credentials and then Create cedentials. Finally, choose API key.
Click on Browser key
Choose a name for the key (e.g. My Maps Key) in the first field, and insert the referrer in the second field. So if your site domain is http://www.mysite.com you should insert *.mysite.com/* then press Create.
Copy the key somewhere and press OK.
Go to Overview and click on (or search for) Google Maps JavaScript API.
Hit Enable.
Repeat the last step for the Google Maps Geocoding API and Google Places API Web Service as well.
If you need the address autocomplete and the real-time directions features, repeat the last step for the Google Maps Directions API also.