Hi salahuddin_66,
First you have to know if your modem is supported under linux. Open a terminal session and enter "lspci -vv | less". If you find a device with "00:xx.x
Serial controller: Motorola ...." you have a real modem. However, if you find a device with "00:xx.x
Communication controller: Motorola ...." you have the 'so called' winmodem. In this case go to
http://linmodems.org/ to see if it is supported.
If anyone of the two appears, then you may have an old ISA modem. You also must identify it. In this case try google with the complete name of your modem or the name of the chip on the board.
Anyway, if you have a hard modem, all you need is to configure ppp, giving your ISP, phone, your account and password and establish the connection. With some sophistication, you can use firewall rules to send to your lan card the ports it accepts and the rest of the traffic to your ppp dial up.
Best surfings.