In order to share the DSL connection (providing that the DSL modem can connect by ethernet) you will need a cable/dsl router.  If you want one of the computers to connect by a wireless connection you will need a wireless cable/dsl router and a wireless network adapter in the computer that will connect by wireless.  A wireless router usually has ethernet connections to allow up to 4 computers to connect by ethernet.  Nearly all current Dell computer models have a built in ethernet adapter.  Wireless network adapters must be ordered as an option or purchased separately.  

Steve

http://www.microsoft.com/windowsxp/using/networking/learnmore/russel_02april08.mspx
http://www.homenethelp.com/connection-sharing.asp
http://www.theeldergeek.com/quick_guide_to_simple_file_sharing.htm
http://service.dell.com/dell/topic/1,,33732+30+26124,00.html

Some reading for you tonight.