Click the Right Buttons
To upgrade your Mac to Tiger, you must boot off of the Tiger installation DVD. The easiest way to do this is to insert the DVD into your DVD drive and then double-click the "Install Mac OS X" icon that appears. You can also boot off the disc by inserting the DVD in your drive, restarting your Mac, and holding down the "c" key as the computer boots up.
Booting off of the installer disc takes significantly longer than a normal boot, so don't be alarmed. Once the computer is booted up, you'll be confronted by a series of dialog boxes that ask you to choose your operating language, agree to the standard user agreement, and select the hard disk on which you want to install. You can install Tiger on an external drive and boot off of it later, but installing the upgrade on your internal hard drive is probably more useful. (No, installing an OS onto an external drive does not mean that you can hook that drive up to a monitor and have a functioning computer.)