Q: New iMac wifi not working

Try the basics first:

   Change your router channel number.  Most times this works & is all you have to do.

Disconnect & reconnect your modem.  unplug it for about 10 seconds.  Plug it back in.  Do the same for Apple’s routers.  Wait for everything to reboot.

System Preferences>Network

Click the Assist Me button.

In the next window that pops up, click the Diagnostic button & do the necessary.

Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer.   See these basic networking KB Articles:  http://support.apple.com/kb/HT1401 AirPort troubleshooting guide

http://support.apple.com/kb/HT2712 Using network locations in Mac OS X

Manually provided DNS server addresses are higher priority than DHCP's

http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting

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What to do when you can't connect to the Internet

Also, run the Airport Utility app which is located inside the Utilities folder.

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If using a  Linksys router, contact LinkSys Customer Support and/or post in their forums.

If using Apple's Airport, please re-post over in one of the AirPort Forums.