You will need to know something about the USB dongle. By your description, it sounds like it is a serial port dongle. There are two most commonly supported versions of this category of device, each based on the chip used in the dongle. One is a FTDI chip, probably the most common, and generally well supported in Linux. The other, I cannot remember right now. In any case, there will be a driver that needs to be loaded. In the case of the FTDI chip, the driver is called 'ftdi_sio', and it requires 'usbserial', and 'usbcore'. If your system was configured in the usual way (and assuming the FTDI dongle type), loading the driver may be as simple as
Further suggestions are somewhat pointless without knowing more details about your dongle. If it is in fact a serial dongle but the dongle is an unsupported type, it may be simplest to get a different one, which is supported. These are quite common and generally not expensive.