Simply use Windows Explorer and find a file on each computer...it doesn't have to be the same file name. Right click on that file and choose properties. Under properties go to the share tab and mark Share this file on the Network. If you want the other party to be able to manipulate the contents of the file, there's a box for that too.
That might not be what you want to do...how much security is there on the network itself. Are you the only person using the network...is there anyone else with access that you don't want to see your private files?