I would suggest checking out the small business/start-your-own business section of your bookstore/library. There are a lot of books on how to write a business plan for cafes and small food establishments. You can apply for a small business ID/tax number with your state, I believe.
Also, I would see if there are any other tea rooms in your area. See what their theme and target customer are, and if you want the same audience and feel to your place.
And, of course, learn about all you possibly can about tea! Try several brands of tea, and see if your favorite brands/type or retail outlets have wholesale divisions (adagio's is http://www.adagioXL.com
). Once you have a tax number you'll be able to go to all sorts of events and trade shows to get a feel of all the products that are available to shop owners that you may not necessarily find online or what have you. But yeah, figure out what kinds of teas you want to offer, and if you're having food served, what kind of food, atmosphere, flavors, etc you want to create.