This is a pretty good&easy test for wireless functionality (as root probably):
Or if your wireless device is not wlan0 but something else (like eth1), change the above command accordingly. It should either print out information about nearby access points (wireless card and it's driver are working all right) or tell you that the interface does not support scanning or there is no such interface (which usually means that the driver is either not working or it's not there).
After that..well, get the driver and re-try. To find out which driver you need to have, see what chipset the card is using. This might provide good information about that:
After you known what driver (kernel module) you need, see
and check if it's loaded already or not. Probably not if scanning failed. Also see (as root)
modprobe -l |grep modulename
where you replace 'modulename' with the module name your card needs, and see if it's there. If it is, load it with modprobe, if not, you need to install it.
EDIT: after you get the driver working and see that scanning works, you can just use your favorite graphical (wireless) network configuration utility to connect to your network(s). These are usually found in your desktop environment's menu or panel, at least in KDE/Gnome. There are command line utilies as well, like wpa_supplicant, but if you use graphical desktop, you can just as well use the nice front-ends they provide.