It has been a long time since I made rootbeer from scratch and back then all contained sassafras. A good, simple recipe contained sassafras root or bark, sarsaparilla root, and oil of wintergreen or sweet birch. I don't remember the proportions, but with only 3 ingredients to work with, you should be able to get it right by trial and error. All of these herbs should be available at large health food stores as "herb teas." Also, oil of wintergreen should be available in the "aromatherapy" department. Otherwise, there are online sources. Make sure that the oils are USP or food grade. Typically, essential oils are used at a rate of less than 1 drop per gallon so don't poison yourself with more. Add essential oils only after you have removed herb debris, otherwise, the devris will absorb the oil. Essential oils have low water solubility but a few minutes in blender will usually dissolve them.
Actually, I recommend using all essential oils. For sassafras free, see first formula here:
http://www.root-beer.org/modules.php?name=Forums&file=viewtopic&t=121&sid=2f2df0590cb4672c6908032bd472f72a
You may not have access to all these ingredients, but it should give you ideas about creating your own formula.
I recommend that you carbonate in a seltzer bottle instead of using yeast.