Qualifications
The International Tour Management Institute (ITMI) is a well-known travel guide training organization based in San Francisco. They offer a two-week training package and an annual symposium. Attendance here isn't required—plenty of tour guides have never even been to San Francisco—but it's an option.
In general, qualifications will vary. Some gigs may require a formal education, while others just want you to have some neat stories. Often, being a tour guide is a transitional position, a first step to something greater or a filler job while switching careers.
If you just want to be a tour guide, we wouldn't recommend going in for a $100,000 education. Plenty of tour-guiding jobs may not require a degree, but they'll demand that guides at least possess a certain amount of knowledge about the subject at hand (source).
Bottom line: you need to be knowledgeable.
Other bottom line: you need to be pretty good with people. If you have the observational skills of a reasonably intelligent goldfish, you've probably figured out that humans can be pretty weird. As a tour guide you'll be interacting with a whole bunch of them. You need to be able to think fast, because there's no way to predict what the people in your group will ask, say, do, touch, or lick.
P.S. Why did you lick that?