Hoo... golly.
1. What computer is it?
2. Is it Intel? Click the Apple logo and choose About This Mac. If it says Core Solo, Core Duo, Core 2 Duo, or Xeon, you're good.
3. Is it running Leopard? If so, Applications, Utilities, Boot Camp Utility, follow the instructions.
4. Is it running Tiger? If so, go find the Boot Camp Beta, set your clock back before December 31, 2007, and follow the instructions.