In order to redirect your website to be opened through HTTPS, you should add the following rewrite rule in your .htaccess file:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule .* https://www.domain.com/%{REQUEST_URI} [R,L]

This will redirect your domain to https://www.yourdomain.com. If you wish the redirect to work without www, you should remove it from the rewrite rule:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule .* https://domain.com/%{REQUEST_URI} [R,L]

Related posts:

1. Redirect subdomains to domain