Cilick on the apple in the top left and click "About this Mac". Click more info at the bottom, this will open an app called system information. Click system report and in the side bar scroll down to Network > Wifi. Click on that and look to se if there is anything listed under the "Interfaces:" section. If there is, this is a software problem that can be fixed pretty quickly, if there isn't, you might need a new AirPort card.