I'm doing this from memory; don't have a Windows 98 box to hand.
Start -> Settings -> Control Panel -> Network.
click on TCP/IP -> <your network card>
then Properties.
(note: If there is only one item TCP/IP click this instead, it will be like this if there is only one instance of it).
If your network card is set to obtain an address automatically (via DHCP), there's not a lot you can do, otherwise this is where you set the information.
Note that if you set the information incorrectly for your network, you will not be able to communicate with other PC's on the network or access the internet.
Be sure you know what you are doing before you change anything.
John