If you work with the Apache web server and can change its configuration, follow the Apache documentation:
<VirtualHost *:80> ServerName www.example.com Redirect "/" "https://www.example.com/"</VirtualHost> <VirtualHost *:443> ServerName www.example.com # ... SSL configuration goes here</VirtualHost>But you also asked if you can do it in a .htaccess file. In that case you can use Apache's RewriteEngine:
RewriteEngine OnRewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L]