How to Set Wi-Fi Network Priority in Mac OS X

1. Open System Preferences from the  Apple menu and click on Network
   
   
   
   
2. Click the Wi-Fi network
3. Now click on Advanced and then choose the Wi-Fi tab
   
   
   
   
4. Click on the wireless network you want to connect to primarily and drag it to the top of the list, arrange the other networks by priority
   
   
   
   
5. Click OK to save the changes and close out of System Preferences

Whatever the topmost wi-fi network is will be the first to be joined, assuming it’s available. If the topmost network isn’t available the next one down will become the preferred network, and so on.