Start at the beginning here. Do you already have a WiFi router and network setup? If not, get one, will have instructions how to connect it i the box. If you want to know how to setup everything you have on the same network, do some web searches for "home lan setup" and also at the device manuals.
Then get a WiFi USB connector for the desktop, way easier to install than a PCI one. This is a good one http://www.newegg.com/Product/Product.aspx?Item=N82E16833704045