A retired computer engineer might be able to write on all sorts of computer-related topics. However, a narrow-focused story on personal computers for businesses might find a better market. No matter what your field, the basic concept for establishing a successful writing business is to sell material to trade publications or directly to businesses concerned with the area of expertise you have. Forget fiction, and forget most of the large mainstream newsstand magazines. Times that's about four double-spaced pages.
Short articles done quickly and easily, with minimal research, can be very profitable.
Once you have determined what your skills are and what your subject is, you must decide what areas of the writing business you want to pursue. There are several avenues that can be explored. Freelance-article writing for trade publications is how most people start out. A visit to a large library, coupled with some research in Writer's Market and Bacon's Directory will yield a large number of publications interested in your type of material. You choose a topic, write about it, and send it off to several publications in the hope one will buy it.