Set Up Sharing Files On Wireless Network

I have a home wireless Linksys network, with 3 computers. Its intended purpose was to allow all 3 computers to connect to the Internet via an 11.0 Mbps internet connection via cable TV, which has been successful. However, I have never learned how to have one computer send info to another within my little wireless home network, a feature I would like to have, but which I have never explored. I have recently been messing with shared folders, placing two of the three computers on the wireless network, but have found it elusive as to how to put info in a folder to allow it to be shared by the other computer. I am unsure as to whether I have properly put each computer on the network. (Yes, I know I could put files on a disk in the A or D drive, which I have done, but I was of the impression that I could do things more efficiently this way, instead. Indeed, I would like to be able to download entire programs from one machine to the other. But lets keep it simple.) What I have done so far simply doesn't work.