For Mac OS X 10.2

1. Press the power button.
2. Right after you hear the startup tone, hold the Shift key. The Shift key should be pressed as soon as you hear the startup tone,not before or after it.
3. let go of the Shift key when you see the gray Apple and progress indicator (looks like a spinning gear).

Mac OS X 10.2 to 10.3.9

you will see Safe Boot on the Mac OS X startup screen

If you want to change your startup on your Mac just follow these steps:

Hardware

The first thing to do is to check your hardware.If you have an old Mac it means you have old parts.The easiest way and cheapest part to upgrade is your RAM.

Wasted Hard Disk space

Take a look in your applications folder.Determine whether you need all those applications.Get rid of all the programs you feel aren`t worth keeping.