Setup: You have a Moodle installed
Problem: You and want to setup https only for entire website.
NOTE: You need to enable HTTPS/SSL your website first !.
Examples:
http://www.iis.net/learn/manage/configuring-security/how-to-set-up-ssl-on-iis
http://www.apache.com/www/how-to-setup-an-ssl-certificate-on-apache/
Solution:
Locate config.php in web folder.
Look for the line that shows:
$CFG->wwwroot = 'http://website.com';
Change the http:// to https:// like the following:
$CFG->wwwroot = 'https://website.com';
If you prefer to use https only for login process you can do that in Settings > Site administration > Security > HTTP security.