56k external modem

You need to have:
1) the modem itself
2) one null modem cable
3) the power adapter of the modem
4) the driver disk of the modem
5) a telephone line cable

If you do not have the null modem cable, you can buy it from computer accessories shop, it depends on the connector type of your modem and your computer. Usually it uses one of the ff three types of cable:
a) DB25 to DB25
b) DB9 to DB9
c) DB25 to DB9
From my previous experience, it usually is of DB25 to DB9 type.

There are two types of telephone jack, the British Style or the American Style.
Make sure you got the right one. Again, it depends on your modem and your telephone line outlet.

After you got the proper cables,
1) connect all the cables to the back of your modem,
2) connect you modem's data cable to the COM port of your computer,
3) connect the telephone line cable to the telephone wall outlet,
4) connect the power adapter (with properly-set power setting) to the power supply,
5) then power on your modem.

Go to your computer's Control Panel, click on Add Hardware, Add Modem,
then let the computer detect it. Upon detected the presence of your modem,
it will ask you to supply the driver, plug in the driver disk to install it.
That's all

install driver first, then connect it
two differents COM port and USB port
COM port~ ensure your BIOS is enable it
USB port ~ plugin is ok