There's nothing to calculating how many watts your PC will need to operate properly. You total up the power needs of your processor, your motherboard, your video and sound cards, and everything else that goes inside your computer, round up to the capacity of a decently-priced power supply, and there you have it. Of course, you have to know the power needs of your processor, your motherboard, your video and sound cards, and everything else that goes inside your computer, but what's life without its challenges.
Actually, you don't. The manufacturer's web site will almost certainly include those specs. So will the sites for many stores selling the units.
But there's an easier way: Use a power supply calculator. These web sites, usually free, let you select components, then calculate the total. One problem with all of them: They don't list every component under the sun. If they don't list, say, your video card, pick another in about the same price range and you'll probably be close enough. Remember, this is all approximate, anyway, so it's best to err on the side of too much power.
Here's what I think of three such calculators, in order of preference.
Newegg: This one is simple and direct, and if it errs, it errs on the side of caution, recommending a too-big supply over an inadequate one.
eXtreme: More detailed than the others, it offers more components and asks more questions. Not surprisingly, it can intimidate the novice user. And according to some folks on the Answer Line Forum, it tends to guess low, which can run into problems.
It's worth noting, however, that the US$2 Pro version of eXtreme's calculator can calculate not only how many watts you need total, but how many of those you need in 12, 5, and 3.3 volt form.
Journey Systems: Once well regarded, Journey Systems calculator is now quite out of date. They know it, and are promising a new one.