The more information you can give the dressmaker, the better a job she will be able to do.
A pattern can help show construction details that you like and if you already own the sewing pattern bring it along. However, I would not purchase a pattern in order to show her what you want because it might not even be usable.
If I were going to take a sewing commission, I'd want the person to bring clear photos of garments they liked, preferably ones which showed the details of construction. If they brought a pattern, that would be fine too. I would then ask the person questions about what specific elements they liked or disliked, especially if there were differnces between the garments. Sometimes there are multiple ways to construct a garment that all give you a similar finished look.
I'd ask them what colors they liked and what fabrics they liked, again based on the photos that were shown. If I were an actual professional, I would be able to recommend different fabrics that would work for the design they liked and explain how the various choices might affect the final outcome (drape, etc). Ideally I'd even have swatches of fabrics for the person to touch and look at.
Your seamstress should be able to have this sort of dialogue with you. Getting a dress made is just like targeted shopping. For your first information, all you need is to know when and wear you want to wear the dress and have some photos/drawings/etc of designs that you like. The final product may be very different from your initial vision, but it gives you a place to start!
Good luck and be sure to post back after your first meeting with any progress updates.
