Plug directly into the back of the wireless router with an ethernet cable. Open explorer or whatever you use and put 192.168.1.1 in the address bar. That should open the router utility. Hopefully, it is not password protected. If not, the user name should be "admin" with no password. If you have a combined modem / wireless router all in one, you may have to type in 192.168.1.254. (that's a period at the end not another dot!

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If you can get in, go to the wireless tab and look for security. You may be able to see the code, but it may show up as a bunch of dots. If that's the case, set the security to WEP and use your 10 digit phone number as the key. That way you won't ever forget it. Of course, the next time the other users log in they will be asked for the key, too.
Good luck. It's really not as complicated as it sounds...