Two ways:
First, go to the Start screen either by pressing the Windows key or by moving your mouse pointer to the lower left hand corner and clicking, then simply type on the start screen. Win 8 interprets anything you type while on the start screen as a search
OR
move your mouse pointer to either corner on the right-hand side (or swipe in from the right if you have touch) to open the Charms menu, and select the one that looks like a magnifying glass. This will open the Search function and give you a search box