Over time you may wind up with several printers set up for your Mac. This results in a list of available printers every time you go to print. If the default (the first one selected) isn't to your liking you can always change it. To do so, bring up System Preferences and go to Print & Fax. Now right-click (you can hold the Ctrl key down if you don't have a 2-button mouse) on a printer in the list and choose: Set default printer. There you go!
Note the plus and minus signs below the list of printers. If you're just getting started, click the plus to add a printer. If you've sold a printer, click the minus to delete one after selecting it. Apple has an excellent 101 of their own about setting up a printer on your Mac.
Want more tips and tricks like this? Visit TUAW's Mac 101 section.