Google Maps geocoding library
To be able to use Google Maps geocoding library on your site you need to get an API key. First you need to create a new project in Google Developers Console. You may do so at the following URL:
https://console.developers.google.com
After creating your project, make sure the Google Maps Geocoding API is one of the services that your application is registered to use. To do that you will need to click on your project name to select it. In the sidebar on the left, expand APIs & auth and click APIs. In the list of APIs select Google Maps Geocoding API and make sure the status is ON for the YouTube Data API v3.
Now you need to create your API key. In the sidebar on the left click Credentials and then click on Create new key. When prompted for the key type, make sure to select Server key. You will be asked if you want to limit this key to certain IP addresses for added security. This is optional and you may leave it blank for now and edit it later if you like.
Once created, make a note of your new API key.
Now go back to Google Maps settings page on your site and fill in your API key in the appropriate text box.