Instructions from google 


https://developers.google.com/maps/documentation/javascript/tutorial


Obtaining an API Key


All Maps API applications* should load the Maps API using an API key. Using an API key enables you to monitor your application's Maps API usage, and ensures that Google can contact you about your application if necessary. If your application's Maps API usage exceeds the Usage Limits, you must load the Maps API using an API key in order to purchase additional quota.

* Google Maps API for Work developers must not include a key in their requests. Please refer to Loading the Google Maps JavaScript API for instructions.

To create your API key:

  1. Visit the APIs Console at https://code.google.com/apis/console and log in with your Google Account.
  2. Click the Services link from the left-hand menu.
  3. Activate the Google Maps JavaScript API v3 service.
  4. Click the API Access link from the left-hand menu. Your API key is available from the API Access page, in the Simple API Access section. Maps API applications use the Key for browser apps.

By default, a key can be used on any site. We strongly recommend that you restrict the use of your key to domains that you administer, to prevent use on unauthorized sites. You can specify which domains are allowed to use your API key by clicking the Edit allowed referrers... link for your key.