Welcome to the forums!
First thing, Fedora 12 is old, obsolete, and unsupported. You can download the current Fedora here: http://www.fedoraproject.org
Second, you need to figure out what kind of graphics card you have. Go to the Terminal (look under Applications, System Tools) and type:
You may need to install the proper driver for your video card, impossible to say without knowing more about your hardware.