I'm assuming you don't keg. I do root beer with Zatarains extract, water, and Splenda. Cheaper than diet coke, without nutra-sweet poison.
If you use yeast to ferment do NOT use glass bottles. It is dangerous. Only use plastic. (If you goof ald let the ferment go to long, exploding glass can be bad for your skin, eyes, etc).
I'd follow the printed recipe that comes with the extract.
Kind of curious about the Splenda thing myself.....I've made root beer for the kiddies, but SWMBO got me thinking about doing a diet soda just for kicks. Do you replace all the sugar with splenda or just a portion of it?
smoke, I'd recomend sticking with a simple kit for the first time. Or, you could probably google 'home made root beer' and find something that way. Let us know what direction you end up going and how it turns out.