Your query is a little vague!
Is it a PCI card or ISA? UTP, BNC, or FDDI? Which OS do you run - W95, W98, or W2000? Which brand is the card?
Assuming PCI, UTP, W95/98, and 3Com or Intel...
First off, copy the drivers from the floppy or CD to somewhere on the HDD. Not only will this make reinstalls easier, it lets you know that you don't have duff media!
Second, open up the machine and put the card in a free slot. PCI allows IRQ sharing but not all cards support it - especially if it's sharing with the AGP slot. Check your motherboard manual for the best slot.
When you restart Windows it should detect the card and ask for the drivers. Point Windows to where you copied the drivers.
Robert is NOT now your mother's brother.
Go into Start, Settings, Control Panel, Network and add in network clients, protocols, and services as necessary. When adding File and Print Sharing, make sure that it is turned OFF for your Internet connection.
If you use TCP/IP you will need to set up an IP address, a host name, and a subnet mask.
If you use IPX be sure to explicitly set the frame type - Auto does not always work properly.
There are some other issues (DHCP, duplexing...), but they are unlikely to be of interest to the home networker. Feel free to ask, though.
If you're going to participate in LAN meets, then be sure to turn Browse Mastering off.
Now you're set.