There are few fixtures that can impact the look and livability of a room as much as a ceiling fan. It can keep you cool on sultry summer days, push warm air down into the room during winter, and even serve as the room's primary illumination if you add a light. The best news: Replacing an existing light fixture with a fan is a simple, one-day task since the wiring is already in place.
However—and this is very important—you can't just hang the fan from the existing electrical box. It's simply not strong enough to support the added weight and vibration of a fan. Presuming you don't have access to the ceiling fan from above, you must either use a specially rated hanger and box to mount the fan between joists or, better yet, screw a fan-rated "pancake" box (so called because it's very thin) directly to a ceiling joist.