You probably can't. If you can get to the admin screen of the router, you can check to see if the passphrase (if you used one) and/or the keys are visible. Otherwise, from the admin screen, you can reset it.
If you can't get to the admin screen, you'll have to do a hard reset back to factory defaults (a small button somewhere on the router, usually requiring a stylus to push, that you have to hold in for a time - say, 30 sec). Doing this will reset ALL of the settings on the router, including logon information to your ISP (if applicable), SSID broadcast and name, etc.
PLUG DIRECTLY INTO THE ROUTER IF NECESSARY to access the internet (using the blue cable that comes with the router, buy a longer one if need be CHEAP), or refer to the instructions, wait for said friend, OR for fun you can go to link below: