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 !.
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.