By default (out-of-the-box), you're given the username "root" and have no password:
Your username does not change (no need for it to). If you don't plan to open your server to the world (not recommended), or share your computer with your little brother, then you don't have to create a password if you don't want to. However, I find it's generally more intuitive to have one when testing various CMS in development, so do this...
1. Turn on XAMPP.
2. In browser, go to http://localhost/security/index.php
3. Change/Add new password under the MySQL section.
Simple eh? The key is to follow the "Security" link when you first open your xampp start page and from there follow the link in step 2 above.