The first thing you should know about adding a Wi-Fi card to a Windows desktop computer is that, as they say, mileage will vary. By this I mean that the physical installation of the card in the computer will depend on the make and model of the computer, and the software installation procedure will vary depending on the make of the Wi-Fi card. Please read the installation instructions that come with your card carefully, and be prepared to deal with recalcitrant screws when opening up your desktop.
WHAT CONNECTIONS FOR WHICH MACHINE?
You should understand what kind of Wi-Fi card (or expansion device) you can use with what kind of computer. This will help you to specify the device you need when you are buying it ...