Error 404 during the installation

Hi,
Sorry for my bad english, but I’m trying to install WordPress and I have troubles with the 5 min proccess.
I did the 4 steps (have I to upload the WordPress folder to the public html, right?) but when I put http://midominio.com/wp-admin/install.php in my browser I just see a 404 error.
I checked my wp-config.php file and I think all is right. What can I do?
Another question: Have I to install WordPress into my public.html before to fill my database connection details?
Thanks!