you need to ensure that your modem is correctly installed, and that it will even work under linux. if it's an internal modem, check linmodems.org
if it is definitely installed, start by using wvdial on a prompt line to establish your conection, use wvdialconf to configure it. once youve got that far, then you can probably use the wiazards and such like to do it all prettily.
I would point out that this is the third time today i've given out the linmodems address, please search this forum for more niformation before asking new questions. thanks