Start with a couple of the light gear lists - on the left side of this site, or there are others
posted here. my profile has a typical one of mine linked (not seriously ultralight, but comfortably so..)

Start by just buying the bits of gear, and practice by driving the car to a local campsite
and overnighting it with your light setup. when you get the minimum you can live with
overnight, take it to the store and buy a pack it fits in.