If you installed wireless-tools then run iwconfig if not then search for the wireless-tools package, download and install it
If you know the channel and essid of your router (assuming you're using a wireless router) then you can use iwconfig to set it. For my example we'll assume you use channel 11 and your essid is 'linksys' ok?
iwconfig eth0 channel 11 essid linksys
Oh yeah, this also assumes your wireless card is eth0 (i.e. your first network adaptor). This has to all be done as root because it will be located in /sbin and that's not usually in the user's path.