wireless modem? Do you have clear wire or something?
Either way...
1. You will need a wireless router. A router connects networks together and can serve as a gateway to the internet. In this case the wireless router will connect the wireless network and the LAN network in your home. You will need to connect the router to the modem for your network to get internet access.
2. Connect the desktop with a cat 5 cable to the router
3. Follow the installation guide with the wireless router to setup the wireless network.
4. Then you can use the laptops wirelessly
Rather simple since it is a home network. So to paint the picture again...
The wireless router will be plugged into the modem. The desktop will be connected into the router via a cat 5 cable. After you set up the wireless network you can use your laptop on the wireless network.
Hope that helps!