Hailed as the ''brave new world" of our future, the internet offers great possibilities for communicating nationally and internationally at the mere touch of a few keys, and with that in mind we need to know a little bit about how the whole system works. The three basic systems of global digital communication are: e-mail, the internet, and the World Wide Web. To participate, you need a computer (most schools and many libraries have them) with a modem-a device to attach your computer to a phone line. Internal modems are about the size of a credit card or small calculator, and fit inside your computer. External modems are placed outside your computer and are connected via cables. In each case, your modem comes with software-a computer program that tells your computer of the modem's existence and sets up the connection.