I did that already. I uploaded WP on folder "foldername". here's my path :
http://domainname.com/foldername/wp-admin
But if I go to http://domainname.com/foldername, I got a "403 permission denied" error. and if I continue on install.php, I got a "404 not found" error.
Is there any relation with the file permissions ? I set wp-admin and it all subfolders as 777.