As the first poster implied, being able to take apart and reassemble an existing machine is a good place to start. Having that machine work correctly after doing so is even better.

Being able to install your OS, drivers, and programs onto a fresh disk is good, too. Then if you run into problems building a new system, you can be relatively sure that the problem is specific to your new hardware configuration, and isn't due to unfamiliarity with the OS setup. Other than that, it's mostly just a matter of doing the research to figure out what hardware is best for you. If you're careful, you'll spend much more time checking out your options than you will actually assembling the beast.