The Nexus S is powered by Google's new Android 2.3 OS (Gingerbread) and went on sale last Thursday. That day, Koush over at the XDA-Developers Forum posted his Root and Recovery Guide for the Nexus S, giving you tools and knowledge needed to root your own Nexus S device.
Click through to the Root and Recovery Guide for the Nexus S.
If that one doesn't work for you, or you're looking for more detailed instructions for rooting your Nexus S smartphone, the Root the Nexus S in 5 Minutes guide from The Gadgets can help you out. It's based off Koush's directions, only with more details for the amateurs.
Both guides assume that you already have the Android SDK installed on your computer.
If you're more of a visual learner, head on over to TheUnlockr for a full written and video guide, which not only tells you, but shows you how to root your new Nexus S.
If you want to learn how to use your new Google Nexus S, then head over to the Nexus S / Gingerbread Guide on WonderHowTo, where you'll find 15 tips for using your new Android smartphone.