You should have a copy of your plat, scrapbook of photos showing other project which inspire you, an idea of your goals and budget, and anything else you feel is important. I actually had an initial meeting with a client and they had built a model, I was impressed with the level of detail. A plat is a drawing prepared by a surveyor which shows the property lines and other site features of the proposed building site. Clients can be hesitant to talk about budget initially, but the more honest you are with your architect about the budget the better equipped the architect will be to assist you.
The initial meeting is also a time for you and the architect to see how you connect with one another. You want to choose an architect you feel comfortable working with, and which understands and respects your goals.