Look up how to install the driver for your usb dongle in linux. There should be a program in the install disk to install the linux driver. If not, go to the usb dongle manufacturer's website and find the linux driver for it. Then, install the driver and reboot your computer. It should recognize that a wifi adapter is installed. If you open terminal and type
there should be something labeled wlan0 which is your wifi adapter. Then, find the network connections settings window and connect to your wifi network.
We will be able to help you more if you tell us the model of your wifi dongle.