(Maybe redundant step
First you need to find out the router's IP address.
Therefore go into System Preferences > Network > Airport > Advanced > TCP/IP while you're connected to the wireless LAN (and www is working).
Under "Router" a number like 192.168.0.1 or 192.168.1.1 or 192.168.2.1 or something similar must be displayed, even if it's not editable due to DHCP in the IPv4 configuration selector.
When you're finished there, go back to the Network Preferences, and select Location > Edit Locations and click the little plus icon to create a new location.
Name it "home" or whatever you want.
Then select Airport again, connect to the W-LAN you have at home, and then go into Advanced > TCP/IP. There make the following edits:
Configure IPv4: Manually
IPv4 Address: 192.168.0.2-255
(2-255 meaning a number from 2 to 255, if subnet mask is 255.255.255.0 and if the router's address has a "0" after 192.168. and before .1, if the "0" is a "1" or "2" or whatever, you have to replace the "0" in the IPv4 address with the appropriate "1" or "2")
Subnet Mask: 255.255.255.0
Router: The one address you found out in the 1st step.
Then press OKAY, and APPLY.
You should be set, but beware, that some Belkin routers have problems with Macs.