First off, congrats on taking this step!
I agree with Tara, if you are buying online, use their own size guides. Invest in a fabric or plastic sewing tape measure to determine your measurements, though, rarely more than two dollars at Michael's or Mart-stores.
The bigger question is what do you think your first outfit would be? I think a skirt and blouse with a cute pair of shoes is a good starting point. You know your own body best, but I think any girl needs a good dark skirt, and finding a top you like is a fun passtime! Oh, and black flats, every girl needs a pair of black flats!
Best of luck, Kara!
-S
PS: I agree that size six in panties would probably be comfy!