First, the PC has to have a compatible wireless receiver. If it does, you need to have information about the wireless network: What type of security, and what the passcode is.
Then, on the new PC, open the Network section and click on View Available Wireless something. Either this will work, or it will tell you that you have more specific software installed that you should use instead. In this case, look through All Programs for your wireless configuration and do the same thing there.
If the wireless network has not been set up to hide its SSID, you will see it and can select it to connect to. You will be prompted for security type and passcode. If the network is protected by hiding its SSID, you will have to type in the SSID to connect to.
I am assuming that this is _your_ network and you have proper access to the access point's information.