If you use Apache or something like LiteSpeed, which supports .htaccess files, you can do the following. If you don't already have a .htaccess file, you should create a new .htaccess file in your root directory (usually where your index.php is located). Now add these lines as the first rewrite rules in your .htaccess:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
You only need the instruction "RewriteEngine On" once in your .htaccess for all rewrite rules, so if you already have it, just copy the second and third line.
I hope this helps.