Depends on the headset. If your bike is reasonably modern, it'll probably be an aheadset. Undo the bolt on top and take off the stem cap, then loosen the stem bolts and slide it off (if your cables are tight, you might need to take the bars off the stem too). Give the top of the steerer a couple of sharp taps with a mallet and the steerer should slide out. There will be some various miscellaneous bits at the top of the heatset assembly, like spacers, rubber washers, split ring and so on - just remember how they go together and keep 'em safe. The only dodgy bit is getting the fork crown race off the fork - this is a little ring of metal which is seated on the steerer tube right next to the crown. With some headsets, this thing is solid as a rock and you can stick a wedge of some sort (screwdriver, chisel, even an axe) between it and the fork crown, and tap away till it comes off. With others, it'll be a fragile little thing that bends if you just look at it, in which case your best bet is to try and buy a new one (not a whole headset, just a new fork crown race). In any case, when you have your fork crown race, whether it's new or the old one, slide it down your new fork and tap it into place, making sure it's well seated, and being careful not to damage it. Then reassemble, and you should be good to go. Good luck!