With a new Mac, you set up the user/pass during initial setup. Since it's used, however, he'll need to boot from an install CD/DVD and use the Change Password utility (or whatever it's called ) in order to change the password of the user that's already installed on the computer.
Once he's changed the already-installed-user's password, he can log in with that. While logged in, go into System Preferences and make a new user with whatever he wants his login name to be, with the password that he wants. Give this new user administrator privileges then log out of this "default" user. Log in as the new user and delete the old one from system preferences.
All Set!
P.S. To anyone who wants to be productive, we should have a "how to make a new user" guide in Mac Guides... I'm WAY too busy to do more than this post.
since it's a new-used or used-new (which is it??? ) none of the docs on it are yours, so I'd just do a format/install anyway. Then you get to set it all up so it's his name and junk instead of the old owner's.
if there were some things you wanted to save, burn them to CD/DVD before doin' all this.