PITTSBURGH — The personal computer is a dazzling device. You can print out a newsletter that, a generation ago, would have taken thousands of dollars in equipment and years of training to produce. You can instantly make changes in computerized ledgers that would have taken hours on paper. But after days and months of such feats, personal computing can get a little lonely. Friends just don't seem to gather around the old computer keyboard. It's time to reach out. An increasing number of PC users are doing just that. They're hooking their machines into telephone lines and exchanging files and messages with people around the globe. Don't wait too long to try this for yourself. There's another world out there, a place where a new kind of communication is taking place. To hook your computer up to a phone, you need a modem. Their special ability is to translate the silent data bits inside your computer into noises that your telephone line can carry. At the other end of the connection, another modem translates the noise back into computer data. This isn't ideal. Eventually, we'll skip the modem and send data directly onto digital telephone lines. But for the moment, if you're patient, a fast modem can meet most communication needs. There are two kinds of modems: internal boards, which slide unobtrusively into slots inside your computer, and external models, which are stand-alone devices. If you're buying a desktop computer and modem at the same time, get the internal kind. It doesn't take up extra desk space and doesn't require its own power supply. If you're buying a portable computer, however, I recommend an external modem. The best ones are PC Cards (formerly known as PCMCIA cards). Make sure your portable computer includes PCMCIA slots that accept the PC Card modem you buy. (Your dealer should configure it for you, in case the machine doesn't automatically recognize it.) You'll also need software to run your modem. The operating systems of IBM-compatibles and Macintoshes usually include basic communications programs. Your modem probably also comes with communications software. These have limited capabilities. On-line services, such as CompuServe, America Online, and Prodigy, provide their own software for hooking into their computers via telephone. If that still isn't enough, several communications software programs are available. I use the industry leader - Crosstalk - on my IBM-compatible, but I've never been enamored with it. Or any other communications software for that matter. The programs are either too hard or not comprehensive enough. Of all the computer activities I engage in, communications is the most persnickety and difficult. The modem is working one minute and having trouble connecting the next. So many things can go wrong that it's difficult to pinpoint a problem. Is it the modem? The software? The phone line? The on-line service? I once spent 45 minutes monkeying around with my mother's external modem, getting nowhere, before I realized the source of the problem: I hadn't turned the modem on. The best advice I can offer is to monkey around in similar fashion. Try having the modem dial a few times. If it still doesn't connect, contact your computer buddy or the technical-support people of your modem or software or on-line service company. If you can figure out the right company to reach, technicians can probably find the solution. Once you have the modem working, the big decision is which on-line service to subscribe to. Do you only want access to the Internet? Or do you want the more organized offerings of an on-line service? So much hype has surrounded the Internet that newcomers often get the wrong idea about it. Yes, you can find something on just about any subject using the Internet. But you could do the same by searching New York garbage cans for discarded magazines and newspapers. If you don't know where to go, it will probably take a long time to find what you want. Even then, there's no guarantee the information will be up-to-date. That's why I recommend that computer novices try an on-line service first. All three major companies - CompuServe, America Online, and Prodigy - organize information much more cogently while also maintaining links to the Internet. Microsoft's new network is also slated to offer Internet access. I like different services for different things. CompuServe is the best way to find out about software. In the past year, I've used it to download new versions of software and ask for technical assistance. America Online has the best news offerings: ABC News to Worth magazine, with everything from American Woodworker to the New York Times in between. On-line companies also give you access to the most important service of all: electronic mail. Electronic mail - or e-mail - is the telegram of the 1990s. You write a message on your computer, address it, and send it off electronically. A network of computers carries it to the right address. When your friend logs onto her service, it tells her a message is waiting. When she opens it, your words pop up onto her screen. Imagine: no paper to fold, no stamps to link, no post office to walk to. Your message won't get there as fast as a phone call, but it sure beats the US mail. I wouldn't want to write a love letter on it; E-mail is about as private as a post card. But it's a great way to take care of things that don't require a phone call. It's also very cheap. Some services charge a small fee to carry the message; others let you send as many as you want for free. E-mail is the best of today's Internet. You will want to try some of the other Internet services. People with similar interests or a particular hobby band together into discussion groups on a portion of the Internet called Usenet. If you can sort through the trash talk, you can find some gems of human communication. The World Wide Web is attracting much attention these days because it's a new way of publishing. Individuals and companies hook up their computers to the Web offering all kinds of advertising and information. The Web is the easiest part of the Internet, because users can point-and-click their way from one Web site to another. The Web is gradually replacing older methods of retrieving information on the Internet. But critics say it doesn't fill the need for interactive communication. A big hit with college students is Internet Relay Chat, where several people, sometimes from all over the world, join an on-line conversation. Of course, the going is a little slow, because everyone has to type their ideas. And because people don't have to reveal who they are, the ideas have few constraints common to face-to-face conversation, such as courtesy and accountability. While today's Internet is interesting, it's the system's future potential that has companies and some government officials excited. Imagine what would happen if you could connect to anyone in the United States and a substantial portion of the developed world. You could hold a phone conversation. Or maybe you'd want to exchange video messages. These activities are coming to pass in bits and pieces on the Internet. One day they'll become simple enough for novices to take advantage of. For the moment, get on line and taste the possibilities. * Parts 1, 2, and 3 appeared Aug. 22, 24, and 25; the series concludes tomorrow.