Find out what sort of licenses you need. Your home state will require some sort of business license. If using a motorized boat, you'll need a coast guard license. If looking to guide in national forests you might need a permit for them, and I believe all national parks require permits. Yellowstone is no longer issuing new permits, for example, and I don't know if similar situations apply elsewhere. If you can't get permits for water you want to guide, try to arrange something with an outfitter who does have a permit.

Also, check regulations for guiding on state lands. In Missouri you cannot guide on Conservation Dept land, for example.

Advertising and promotion are what will get you business. Get a good business card. You can use www.vistaprint.com, but get the cards you have to pay for and jump for the thicker paper. You need either a basic website and a great brochure or a great website and a basic (single-page even) brochure/flyer. My money is on the latter option these days. Look into letterhead or plain old mailing labels. Give some fly manufacturers a call to try to get wholesale guide pricing on flies, especially if you plan to include them in the price of the day.

Also, check your IM.