Readers of this column have frequently been urged to try "shareware" -- software available free on a trial basis from computer bulletin boards. The advice, alas, is not much help to most people, who don't know what a computer bulletin board is.

Even readers who do know may have little idea how to hook up a modem, dial a strange telephone number and "download" a computer program. But computer bulletin boards and commercial on-line information services are one of the wonders of the PC revolution. What has long been needed is a guide to this unique world.

Well, now there is one, written by the ubiquitous computer journalist John Dvorak. His irreverent columns are a welcome relief from the stuffy, puffy fare found in the leading computer trade magazines. "Dvorak's Guide to PC Telecommunications" is a thousand-page paperback from McGraw-Hill written in his breezy style. It costs $50, but comes with a bonus that makes it worth the price.

The bonus is a pair of disks that include Telix SE, a special version of a powerful shareware modem-communications program. The disks also contain a sample of Dvorak's other favorite shareware plus a comprehensive tutoring program on how to use modems, developed by Dvorak and co-author Nick Anis. The program explains all you'll ever need to know about how modems work, and shows you sample screens from leading communications software as well as from some leading bulletin boards. It's menu-driven, allowing you to skip around.

While the included programs and the modem tutor are written for the IBM-PC and compatibles, there is a 24-page chapter for Macintosh users.

You'll need a PC with more than 1 megabyte of disk space -- preferably a hard disk -- to accommodate the programs on the two disks. They are stored in compressed form enabling them to fit on two 5 1/4-inch 360K floppy disks.

Finally, of course, you'll need a modem. The book, helpfully, has a directory of firms that manufacture them.

A few recommendations. Virtually all modems today use the so-called "AT" or Hayes command set. You don't need to know what this is, just make sure you get a modem that uses these commands.

You'll have to choose whether to have a modem that goes inside the computer or stays out. Internal modems are cheaper, but will require an expansion slot inside your computer. You may have to pop your computer's hood to install the modem, but after that, it gets easier. There's only one cable to hook up -- a standard phone line from the modem to the wall jack.

An external modem connects by cable to your computer's RS-232 or serial port. Lots of potential problems here. Some such ports have nine pins, some have 25. You can get an adapter or a special cable to solve the incompatibility, but who wants to bother with that? Besides, if your computer has only one serial port, you may already be using it to connect your mouse or some other device, such as a cable to transfer files from a laptop.

Some external-modem users swear by them because they like the indicator lights on the modem's front panel that tell you what's going on. Usually, however, your communications software will keep you apprised of what your modem's doing.

As for brands, Hayes, author of the industry-standard command set, makes durable modems that do a good job of filtering out noise from telephone lines. They are expensive, however, and most people are safe with a less costly brand. U.S. Robotics, for example, makes solid modems at reasonable prices.

Modems operate at speeds up to 9,600 bits per second (or "baud" in computer slang), but you'll need excellent phone connections to move data that fast, and most services and bulletin boards only support a rate of 2,400. So don't spend the extra money for 9,600-baud, but do spring for 2,400. An internal 2,400 should run about $100, an external about $150.

With your modem installed, you're ready to get the most from the Dvorak guide. There's a directory of bulletin boards all over the country. Also, there are sections on a host of commercial on-line data banks including CompuServe, Genie and the Dow Jones News Retrieval service.

The rest is automatic.

Once you've "logged on" to a bulletin board or data bank and delved into the treasure trove of information and software available there, you and your computer will possess power you may never have dreamed of.

Brit Hume is a contributor to the Washington Post Writers Group. He is chief ABC News White House correspondent and the founding editor of a computer newsletter.