When I first encounter something I do not know or fully understand I usually try to work my way from examples.

You can get example code from here: https://developers.google.com/maps/documentation/javascript/markers#maps_marker_simple-javascript

here an example of setting a custom icon: https://developers.google.com/maps/documentation/javascript/markers#icons

I like SGV format icons because it saves me the trouble of having to setup files and load them from the scripts. From there I can define color, shape, size.