Check out Northern Brewers website. I've brewed their Witbier four times. Side by side tasting at my drinking establishment, NBs was preferred.
NB also gives you the recipe.
Otherwise I also agree with the other suggestion you got about checking out Northern Brewer's recipes and kits. You can buy the kit right from them that will include all the ingredients (all extract if you go that route), and detailed instructions.
As for how to convert from all grain to extract, it's a deep question with as many answers as you have time for. You can apply some simple formulas, or read all about the subject here. For now, as a new brewer, I suggest you just stick with one of the above mentioned extract options.