If you're using Ubuntu, and using a recent version of it (say 8.04 or 7.10), you should have a tool called Restricted Drivers Manager somewhere in the System menu (in it's submenus). Run it - it probably asks for your password at some point - and your graphics card should be listed. Check the box to enable it (you need to click a button to apply) and it should automatically download and install the driver for you - this is the easiest way to get it working.
Second possibility is to open Synaptic package manager (again somewhere in the System menu) and try to find the nvidia driver package there and install that.
Third possibility is to head to nvidia's site, go to downloads, select your operating system and card model and download the file you're told (a .bin or .run file probably). Then you need to close X, the graphical server on which your graphical desktop runs, because the driver insists (or did last time I tried this) it's not running while you "manually" install it. You can probably do this by running
if that won't bring you to a command line login, try other numbers 2-5 (do not try 0, 1 or 6, they're reserved for halt, single-user and reboot). Once you login, use 'cd' command to change working directory where you downloaded the file, for example
See that the file is here:
then make it executable and run it with root privileges (could be .bin, .run or something else - you should know the name)
chmod +x driverfilename_whatever.bin
Then follow instructions. If it says you're missing something, install the missing things and re-try. You can install new packages like this, for example (or with aptitude):
sudo apt-get install packagename
or if you'd like to get back to graphical desktop,
Or if 5 isn't the right number, try 2-4 to get the right one.
After the driver is installed, reboot to make sure the module gets loaded correctly.