You are very lucky to find out you have a wireless router!!!
Anyway I would unplug the internet cable for one of your computers and plug in the usb adapter. Make sure you choose a computer that is within a certain range of the wireless router. Too many walls, or even closed doors can interfere with your wireless signal.
If a disk came with your wireless usb adapter you will be asked to insert it. Sometimes the computer will find it automatically... If you have Windows XP or Vista, this will be the case.
Once you have installed it, you need to find a wireless network.. This page may help
how to find a wireless network to connect to..Hope this helps
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