Does your iPhone sync with another computer? If yes, when you connect the iPhone to your Mac, it'll ask to transfer purchases or erase and sync the audio/video. If no, skip to the next sentence. Once you see the Summary screen for your iPhone, select the Info tab to the right and click the check mark next to Sync contacts with Address Book. Click the Apply button in the lower right and within a minute you should be done. If you have multiple computers or iOS devices you should consider the MobileMe service to sync contacts over the air (OTA) rather than through the sync cable.