Check the links kelly gave you but also Apple Help in your menu. But the most common 'error' in any networking seems to be the sometimes unintuitive action of actually having something to share. Not sure what OS you're using but in 9 (and below) you must select a file, folder, or disk to share. It is so simple that it is often overlooked. Just do a Get Info on what you want to share (on each machines) and select "Sharing" in the popup list. When that is done, you should be able to see something on your network, after all the appropriate settings are completed (mainly setting Appletalk to use Ethernet and selecting passwords for every machine [write those down!]).
Hope that helps, but you probably already did all that!
Jim C.