as of Jan 2017, unfortunately @Adi's answer, while it seems like it should work, does not. (Google's API key process is buggy)
you'll need to click "get a key" from this link: https://developers.google.com/maps/documentation/javascript/get-api-key
also I strongly recommend you don't ever choose "secure key" until you are ready to switch to production. I did http referrer restrictions on a key and afterwards was unable to get it working with localhost, even after disabling security for the key. I had to create a new key for it to work again.