Nothing more than being connected to the internet anyway. In the security settings, make sure the firewall is set to block access, ideally in stealth mode too.
LAN: connected to a router? Is DHCP set in the router, and then check the settings on the Mac.
I assume that you want wifi?
the mac setup uses 128bit wep by default .... most routers use 64 ....
get the key from your router .... open the macy nety thingy and click on th drop down ... which will just say wep
when you drop it you'll see the options