Depends on which model it is. Most of them that age already have a wifi board in 'em. Try turning on wifi from the system preferences app. Or go to the apple menu (upper left corner of the menubar) and check "about this mac." In the dialog box that opens, depending on which version of the OS you have, there's a link to open the system profiler or to get more info, either of which will tell you about all the hardware available as the machine is currently configured.