nisiemouse, the vast majority of women wear the medium, so unless you feel you have much lighter skin the most women I think that would be the way to go. Also, some women order the medium and the light to mix them. I would also suggest the mineral veil, it's wonderful and adds a baby smooth sheen to your face that is incredible. It is also made from minerals and is feather light.
If you've never used barecsentials before, it will take a little practise but it's definitely worth it. Be sure to dip the brush in the powder, then swirl in the lid, tap off excess; then buff on your fact. Just keep repeating to achieve the coverage you want. What's amazing is it never looks or feels like you're wearing a lot of make up. Good luck, I hope it works for you. Remember, QVC does have a money back guarantee. Sharon