To use Google Maps you have to creat Google Maps API key.
To do so:
1. go to: https://developers.google.com/maps/documentation/javascript/get-api-key and click "get started".
2. choose products
3. create a project
4. Set up a billing account. Google Maps is a paid service hovewer you have free $200 every month. This is amount will be enough for small/medium sites to use API for free.