Definitely have someone with lots of showing experience go with you at least for your first few. There are many little things that are easily forgotten, and just general rules of showing, both stated and unspoken that people follow. Mostly courtesy and safety oriented.
May want to work on your horse working around other horses and many people (don't know how yours reacts in those situations) but it's always a good training tool to have under her belt anyways.
Most of all.. Have fun! Shows are exciting, tiring, and cool to participate in :)
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin