If you have the wireless-tools package installed, you can use
iwlist wlan0 scanning
to detect available networks. (Note, this is assuming your wireless device is wlan0, if it is under a different name, just use that instead.)
Once you see the proper network, you can use iwconfig to set the essid, key, channel, etc. I only needed to set the essid and key for mine. Like so:
iwconfig wlan0 essid myessid
iwconfig wlan0 key open myhexkeyyy
If you aren't using WEP, you won't have to worry about the key, but iwlist will show you all the details anyway.
The last step is getting an IP. If your router is configured for DHCP, just use dhclient or dhcpcd, and you should be up and running.
Hope this helps.