This really going to depend on whether the modem is a "real" modem (meaning that it has a onboard DSP chip and can send properly decoded data across the serial connection) or a "winmodem" (meaning that in order to get the cost of the modem cheaper, they have left off the DSP chip and emulate in software on the computer). If you have a "real" modem (and all external modems are real modems), you can plug it in and it should be recognized as a new serial port (general COM4 or higher for PCI internal modems). Then all you need is your favorite comm package or program to use it, and possibly an appropriate getty entry in the /etc/inittab file if you want to use it for inbound connections.
Winmodems are trickier. There was a time when Winmodems did not work at all because the DSP-in-software stuff was propriatery to Windows. This has changed somewhat (I believe, because I always avoid Winmodems with Linux) and some Winmodems are now supported under Linux. It will greatly depend on the modem of your modem.
Hopefully this will point you in the right direction.