How hard is it to put a wireless card in a pre built computer?

How hard is it to put a wireless card in a pre built computer?Is there any where you can recomend me to look?

Do you mean a PCI wireless network card? As long as your PC has a free PCI slot and there is room in the case (there should be if a slot is free) the installation is easy. It takes at most 5 minutes.

USB dongles are less reliable. A built in PCI card with an antenna is better.