I took the plunge and began work as a full-time instructor in January. My co-workers and I joke about having taken "the vow of poverty", but if I had to do it again ... I would.
Be prepared to work hard. This job isn't 8-5. We work weekends. Always keep an open mind. Get as much experience as you possibly can. Learn from as many teachers as you can get to talk you ... formally and informally, on the range and off. Work on your public speaking skills. Learn about how adults learn. Take instructor courses from an established instructor or company. Did I mention to keep an open mind?
Frankly, though -- if you've ever taken a newbie to the range or if your circle of friends/family looks to you as the "gun guy", then you've already stepped into an instructor role. The more you teach others, the more you will realize you need to learn (I promise.) Keep working on it and keep learning.