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!
The Edimax WiFi adapter is popular because it's small, cheap, and most common pi distros come with the drivers already installed. Setting this up via the command line is simple.