You will require a computer and a medium containing a Linux distribution. Most of this guide applies to all Linux distributions - and UNIX in general. Apart from time, there are no further specific requirements.
The Installation HOWTO contains helpful information on how to obtain Linux software and install it on your computer. Hardware requirements and coexistence with other operating systems are also discussed.
An interesting alternative for those who don't dare to take the step of an actual Linux installation on their machine are the Linux distributions that you can run from a CD, such as the Knoppix distribution.