RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://www.example.com/$1 [R,L]
Please, note that the .htaccess should be located in the web site main folder.
In case you wish to force HTTPS for a particular folder you can use:
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteCond %{REQUEST_URI} somefolder
RewriteRule ^(.*)$ https://www.domain.com/somefolder/$1 [R,L]