You need a router! Without some intelligence that routes packets from one source[Internet] to another [LAN], you will not be able to share that connection. Most routers comes w/ a switch integrated into the router.
Unless you are getting two ip addresses from the dsl modem you MUST use a router. A router will enable more than one computer on a network to access the internet at one time by using DHCP to hand out ip addressess for the new network, called NATzZ. A switch just directs packets to the right computer.
Most routers you buy at electronic/computer stores have a built in switch to allow more than one computer to hook up to the router. Normal routers usually only have 1 port on them.