Go to your local computer store, or get on the web and find a copy of Linux. You can buy a copy starting at around $30 at major computer retailers, or you can get a copy for the cost of media ($5-10) at somewhere like
CheapBytes. You can also download it for free if you have a fast network connection. There are versions for Intel (most common), Alpha, PowerPC, HP PA-Risc, Sparc, and others. You can use just about any hardware from a 386 to the latest Pentium 4. Unlike Windows, Linux does not require you to have the latest and greatest hardware. I happily ran Linux on an original Pentium-133 until just a few weeks ago -- it ran for approx. 2 years, with no unplanned downtime.
I would recommend starting with RedHat, purchased at a retail store. RedHat is one of the most newbie-friendly distributions, and your $30 will buy you a printed manual and some basic technical support.
Check the FAQ section of this message board to find some info on introductory books you might like to read to help you get started.