Desktop PCs (unlike laptops) don't usually have wireless adapters built in. You can get wireless adapters as PCI cards or as USB devices. click here for a selection of what is available.
Even if your broadband package includes a wireless router you can still plug into the router using an ethernet cable as the routers usually have 4 ethernet ports in addition to the wireless facility.
A little more information on the package you are looking at could be helpful.