Google map is one of the best option to indicate the places on the map, now a days people show the marker of their address on their web-site using google map. if you want to add a google map in your webpage then you need to generate google maps API key for your site, you need to get this key from google and you have to mention your domain name, which allows you to display google map in your webpage. please keep in mind that google API key is unique for each domain, it means you can not use same API key for sub domain as well.
If you have google account then you get this API kay, because google API key attach with your account. if you have account then login to your google account then you get google API key from below url
Google provide this service absolutely free, there is no limitation on map load(map previews). if you are working with the sub domains and you need google map on every sub domain then you need to create the Client ID for all your sub domain web applications.