Click this link: http://www.facebook.com/about/timeline
This is Facebook's "Introducing Timeline" page, and click "Get It Now" to activate the feature. Or, you can wait to see an announcement for Timelines at the top of your profile page.
Once you make the switch, Facebook will generate a Timeline on its own, but won't immediately publish it. You'll have seven days to make adjustments -- say, to remove references to jobs you hated or significant others you'd rather forget. After that, there's no going back to the old profile.