Typography is, quite simply, the art and technique of arranging type. It's central to the skills of a designer and is about much more than making the words legible. Your choice of typeface and how you make it work with your layout, grid, color scheme, design theme and so on will make the difference between a good, bad and great design.
There are lots of typography tutorials around to help you master the discipline. But good typography is often down to creative intuition. Once you're comfortable with the basics, visit some typography resources to investigate font families and discover some font pairings that are made for each other.