WifiInfoView is a free software you'll need to download for this purpose.
Once installed, click on "Channel" to sort by channel. In this screenshot, channel 11 is the most used channel. We should make sure to choose something other than that!
Hold the Option key and click the Network icon in the system tray. Choose "Open Wireless Diagnostics".
Ignoring the wizard that popped up, click "Window", then "Scan".
You'll see a summary of the channels used and the best channels for your router based on the frequency. Simply choose those and you'll be in good shape!