Whats the model identifier of your machine?
Have you taken any troubleshooting steps? if any what are they.
Problem could be with the WiFi card, the antenna or the OS..
Start with the easiest and run a software update to ensure that your system is up to date.
Then check under -> About this Mac-> More Info -> Network -> Airport
for the wifi Firmware Version it should be something like "Broadcom BCM43xx 1.0 5.100.198.104.5"
You can get it fixed by any local apple retailer if you still have AppleCare otherwise, If firmware and OS is up to date, then have a look inside only if you are comfortable taking your machine apart.
Just a bit tricky but pretty much straight foreword, like taking apart any laptop. The wires to the WiFi card connecting to the logic board are most likely problem check the connections first and look for two wires with gold endings attached to the airport card.If all is well at this point the its defiantly the card that needs a replacement, or you can use a 3rd party USB wifi card that will probably be cheaper.