Go into System Preferences and locate the Desktop & Screen Saver section. Under Desktop, you can choose from the included images, or select one from your iPhoto library or a custom folder. You can even specify times changes if you like, to keep things interesting. In the Screen Saver tab, you can assign hot corners to start your screensaver as well as showing a clock whenever it's running. In the Security & Privacy tab of System Preferences you can choose to require a password to stop the screen saver, which is still the simplest way of securing an unattended Mac - extremely useful in an office environment or one in which kids are apt to fiddle.
