You can share the broadband connection with a decent piece of kit which integrates the LAN connection, ADSL connection and wireless as well - a so called ADSL router.
If you want wires, then the cost for the kit is cheaper, and you'll probably find your computers have Ethernet cards already built in.
If you DON'T want wires, then you'll deffo need a wireless part somewhere - either integrated or an additional cable-linked bit. Each PC you wish to be wireless - probably one, since you can run a short cable to the one in the same room as the router - will need an addin card with a wireless adapter.
Either way, the principle of then connecting them together to share printer/files is the same.
Example ADSL routers, with all the relevant kit are:
Netgear DG834G
Belkin 802.11G ADSL router
Linksys equivalent