Q: How to set up wifi on my iMac

Have you tried unplugging modem and router then rebooting (modem first then router)

See these Apple Support Topics:

https://support.apple.com/en-us/HT202222

(This was written for older OS X systems but updated in 3/18/2011 and is a pretty good troubleshooting guide.

Basics connecting to wi-fi:

https://support.apple.com/en-us/HT201735

Please delete wifi setting and create a new one.

What does Wireless Diagnostics show?

Wireless Diagnostics

Wi-FI connection drops

Wi-Fi Problems in OS X Yosemite

Wi-Fi Problems in OS X Yosemite (2)

Wi-Fi Problems in OS X Yosemite (3)

WiFi Potential Temporary fix - Yosemite

Also try turning off Bluetooth.

WIFI AND BLUETOOTH FINALLY SOLVED, NO REALLY - YOSEMITE 10.10

If your Wi-Fi source is capable of generating a 5 GHz Wi-Fi network, try that to help avoid conflicts with Bluetooth.