I had a setup kinda like this for a long time, only with a wired connection instead of wireless. Really, it’s not that much different though.
What you are going to end up with is two networks. The first network is your modem, which connects to the internet. The second network is going to be a local network which only exists inside your house.
The first network is already mostly set up, so not much to do here. What you are going to want to do is set up what is called internet connection sharing. If your operating system is windows ME, 2000, or XP, this is just a check box that you enable under the network properties for that link. You need to have your second network set up already before you do this.
Your second network (the local one inside your house) is going to be a bit harder to set up, but it’s still relatively easy. You need to add a network card to your desktop, if it doesn’t already have a network port on it. This is your network. You are the network administrator. Every machine on the network has to have a unique IP address. The important thing is that your desktop is going to be the “gateway” to the internet in your house. Once you’ve installed the network card, you’ll need to go under network properties. You can’t automatically obtain an IP address, since there’s no one to get an IP address from on your network, so you’ll have to set it yourself. You need to use the IP address 192.168.0.1 and a netmask of 255.255.255.0. If you manage to set this wrong, windows will change it for you when you click on the modem’s network and enable internet connection sharing.
Now you’ve got your desktop all set up. He has a connection to the internet, and he has a local network that can also access the internet. Now you need to set up your laptop.
In order to connect to your network, you need to connect your wireless router to your PC. Normally, there’s a switch or a hub that all the network things connect to, but since you’ve only got two things you can connect one directly to the other. This is also important: There are two types of ethernet cables, “straight through” and “crossover” cables. If you go to a hub or a switch, you need “straight through” cables. If you go directly from the router to the PC, you need a “crossover” cable. Then set up your router and the network card in your laptop according to the manufacturer’s instructions.
On your laptop, you can probably use DHCP to automatically obtain an IP address, but I personally always use a static address just to make things easier to manage. If you use a static IP address, it needs to have an IP address of 192.168.0.x where x is some number between 2 and 255. The net mask will be 255.255.255.0, and the gateway will be your desktop’s address on this network, which is 192.168.0.1. If you use DHCP it should automatically give you pretty much the same thing.
If you want to be able to share files from one computer to the other, you’ll need to enable microsoft file and print sharing. Make sure that both computers are configured to be in the same workgroup (in the same place as where you identify the computer’s name) and that they each have a unique name.