On Godaddy, you can install WordPress directly with their community board application. Everything is pretty much done by the server itself.
About all the usernames and passwords, you have a pair to access your account (setup / billing thru godaddy.com), then you should have another pair for your hosting (ftp / uploads), one for each database you created (used by WordPress during installation) and a last pair for your WordPress admin / user account. That’s the one you want to use to access your blog. I hope that it helps you.