On a basic Moodle site, it will be simple to set up https. Simply edit config.php and change http:// to https:// in $CFG->wwwroot.

However if you are using a proxy or load balancer, depending on your setup you may need to set $CFG->sslproxy to 1, and not use SSL on the Moodle server. Then the load balancer or proxy server can communicate directly to your Moodle site, but serve to the clients over SSL.

Source: Transitioning to HTTPS – MoodleDocs