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 On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L]