Assuming that you have a recent base station and Mac computer.......
Open Finder > Applications > Utilities > AirPort Utility
Click on the picture of the AirPort router
Click Edit in the smaller window that appears
Click the Base Station menu at the very top of the screen.....up near the Apple icon
Click Show Passwords....and your password(s) will be revealed
Click on the image below to enlarge it.
IF......you have an older AirPort base station or Mac, we'll need the model number of the AirPort and the operating system that the Mac is running to provide more instructions for the older equipment.