netmeg - 8:18 pm on Apr 12, 2011 (gmt 0)
Well a little more than that. First of all, you want to make a decision on which you want to use. (Take into account whatever good backlinks you have - are they linking to www or non-www? And whatever you pick should be reflected in any other branding you use, from ads to business cards to the side of a truck, or wherever you publish your site address. Pick one and stick with it)
Then you want to make sure that only ONE of them goes into the search engines, so, for example, if you pick www, then you need to make sure the non-www version properly redirects to www - thus if I type in yourdomain.com/about.html it will automatically show as www.yourdomain.com/about.html in the address bar. How you do that depends on what kind of server you're sitting on; for apache it's a fairly simple redirect in .htaccess.
THEN you can go into Google Webmaster Tools and tell them which version you want to use, but as part of that process, they'll ask you to verify both the www and the non-www before they establish a set preference (that's because technically, www and non-www are two different sets of urls, in theory, could be controlled by two different entities - although they usually aren't, but Google is just making sure)
Then you just sit back and wait for Google to sort it out. Depending on how many URLS you have, could be a couple weeks or a few months.