Troubleshooting an IMAC

[HOSED]

I am trying to help a friend out, they may have used Tuneupmymac or other questionable site. I am very new to Apple products and want to start with an online scan for malware, can anyone recommend one or another approach entirely?

If I login to my router via wireless how can I completely remove any trace of the passkey to access it?

Thanks for any advise, the user logins as admin and I do have the password

 

[Eug]

You can delete the WiFi passwords from the keychain.

You might also want to install Sophos anti-virus on his machine by the way.

 

[HOSED]

Thanks for the help, I did not check remember this network and had to enter the passkey again after reboot so I guess I am good. I was not prompted to add anything to keychain. I will install Sophos also. I did look at keychain access and saw no obvious entries related to this.
I also followed the procedure by Linc Davis on this page because I did see MacKeeper popups and entries in the history in Safari https://discussions.apple.com/thread/6442738

is it a good idea to clear the 300 + cookies and if I do will the 40 + stored passwords get cleared also by doing this?