To answer your question...
Women usually dress conservative, i.e. nothing revealing, no sleeveless tops or anything that reveals the back or midriff, no low cut tops, no skirts above the knee. It's like dressing for a school or work dress code, basically, except your applying that code to a dress and not casual wear. If I was you though, I wouldn't worry too much, since you're new, everyone should understand if you make a mistake. :)
Sorry if you got rather overwhelmed here, just this is mostly a forum for people who have lost faith in the Society, and with good reason. They're simply trying to prevent you from making the same mistake they have, before you become so enchanted with the Society that you stop listening to anything but "the faithful and discreet slave." If this is really for you, go for it, but just please remember that a blind decision is a poor decision... and that you can't just look at one side of a story, turn a deaf ear on the other side, and say you know the truth about it, as the Society will soon enough expect of you to do. They may tell you to only listen to them because something else will "stumble" your faith, but ask yourself how strong is your faith if it is easily stumbled by you so much as listening or reading something contradictory to it?