The 'root' user and its password "should" be what you try to access phpMyAdmin with -
i.e. you should try user 'root' and either a null (nothing) for the password - or if you used the XAMPP security option - try "that" password. IF all else fails (next step) there's a method for clearing the root password on MySQL.
As far as Guides - for myself -- I used the LinuxCBT videos series (now available as streamed subscription) to learn the basics of MySQL. The guy's a bit dry, but he sure knows his stuff. There is a LOT to know to get to grips with MySQL.