For Mac OS X 10.4 and 10.5
- From the Apple menu in the upper-left corner of your screen, select System Preferences.
- Click on Security
- Check the box next to Require password to wake this computer from sleep or screen saver.
- Click on Show All on the top of the System Preference window
- Click on Desktop & Screen Saver, then the Screen Saver tab.
- Set your desired time limit for starting the screen saver
- You may also click on Hot Corners to specify where to move your mouse on the screen to automatically activate the screen saver.