Depends on how traditional you want to be. Witbier traditionally has oats as part of the grist. It isn't really possible to use oats without at least a partial mash.
That said, you can make a reasonable approximation of Witbier with only extracts. Wit is a wheat beer, so start with a good wheat extract. I like Muntons; it's 60/40% wheat/barley and ferments rather dry.
Witbier is a light-colored beer. So you'll want to brew this with the "late-extract" method, adding most of the extract quite late in the boil. I use 5 lbs of Dry Malt Extract (DME), dissolving two pounds in 3 gallons of water in my brewkettle. This is what I boil. I add the rest of the DME at the end of the boil.
Bitter with an ounce or so of low-alpha hops like Styrian Goldings or Hallertau or something. Whatever you can find that isn't Cascades or one of the other citrusy, obviously American hops varieties.
Towards the end of the boil, add an ounce each of orange peel and cracked coriander. You can buy dried peel from your homebrew store or simply peel an orange. Some will tell you to use only the zest, but (IMO) you want the pith, too - part of the bittering in Witbier comes from the peel. Coriander you can get at any Asian market; otherwise check the LHBS.
The most important part of Wit is the yeast. Both Wyeast and White Labs make high-quality liquid Wit yeasts. Definitely use one of those.