Let's use NewEgg's website, for example: each component you plan on using will have a 'specs' tab, and you'll see the power consumption/wattage listed. Write them down for every component you're going to use in your build, then total it up. If you plan on future upgrades (like a 2nd video card), add that in your total. Then round up a little. Don't forget to add the wattage for your case fans and any blinky/flashy lights you might throw in your case!
For example: if your total wattage is 510, i'd recommend a 600 watt PSU. If your total is 550, i'd play it safe and buy a 650.
You can get an estimate by adding up the TDPs of your main components plus ca. 100W-150W for safety margin and other components.
You can find many calculators that take your system specs and output an estimate online. Even pcpartpicker has been updated with that functionality http://pcpartpicker.com/ it's a brilliant site for any builder.