Wroaw's Way Wicked Board

the modem gives you internet access, the router allows everyone to share it.

If he's using XP then setting up wifi is very easy but if it's most distros of linux, wifi can be a stupid little fucker. It was quite a pain in the ass to get my Linksys WUSB12 to work on my debian box. <_<

Easiest way is like Sv said...

DSL modem -> ethernet port on router.
computer(s) -> port x on router.

Then set everything to use DHCP.