It all depends on what modem chipset it is (if its supported I mean), if uve read the winmodem how-to at linuxdoc.org and if uve already downloaded the necessary modules.
Without some basic nfo from ure side this is going to be a bit hard.
basically its like this.
1. internal or external?
2. usb/serial/parallel?
3. note the modem's chipset from the box, labels or windoze.
4. look up the chipset in
http://www.idir.net/~gromitkc/20010302a.html for support.
5. download the necessary modules
6. build kernel modules to be loaded
7. pnpdump > /etc/isapnp.conf
8. uncomment correct modem lines, IO address, IRQ's etc.
9. run isapnp to check it works
10. redo 8/9 if doesnt work
*99. decide to buy proper hardware modem...