Security: To set your screen-saver password on a Mac

For Mac OS X 10.4 and 10.5

1. From the Apple menu in the upper-left corner of your screen, select System Preferences.
2. Click on Security
   
3. Check the box next to Require password to wake this computer from sleep or screen saver.
   
4. Click on Show All on the top of the System Preference window
5. Click on Desktop & Screen Saver, then the Screen Saver tab.
   
6. Set your desired time limit for starting the screen saver
7. You may also click on Hot Corners to specify where to move your mouse on the screen to automatically activate the screen saver.