If your distribution (which you should add to your profile information so it was easy to see there when answering) uses a package manager for installing software, like most modern distributions do (like Ubuntu's Synaptic or SuSE's YAST tool), see if there is a package available that installs the nVidia "control panel" (might be called nvidia-settings-something too, not sure). It works if you are using the proprietary nVidia drivers (not necessarily otherwise), and is probably the easiest way to configure your screen(s).
If you don't want to do that or don't find it or something, then either see if your desktop environment has a tool for configuring your screen (resolution; Gnome and KDE have this tool). If that doesn't help or you can't find it, you can always just manually alter your graphical server's (Xorg probably) configuration file /etc/X11/xorg.conf - you need to be root to do that. There you can specify the resolution(s) you want to use; though I recommend installing (if you don't have already) the nvidia proprietary driver and then the nvidia settings utility.
EDIT: see
for information about the configuration file.
The problem is most probably caused by wrong resolution; if your screen is a widescreen (like they nowadays tend to be) you need to use a widescreen resolution; if it's not a widescreen, use a non-widescreen resolution. For example 1024x768 is a non-widescreen resolution whereas 1280x800 is a widescreen resolution (compare 1024/768 to 1280/800 and you'll see the difference). Your 24" screen probably yells for bigger resolutions, but nevertheless keep the ratio correct depending on the screen type (wide or not).
