Choose preferred Wi-Fi networks on Mac
By default, your Mac attempts to join the wireless network it was most recently connected to. The wireless networks your computer has connected to are listed in the Wi-Fi pane of Network preferences.
If you’re in an environment with more than one wireless network, you can set up your Mac to join a preferred network when you start your computer, wake it from sleep, or turn Wi-Fi on.
On your Mac, choose Apple menu
> System Preferences, then click Network.Click Wi-Fi in the list at the left, then click Advanced.
If Wi-Fi isn’t in the list, make sure your Mac has wireless capability, then click the Add button
at the bottom of the list. Click the Interface pop-up menu, choose Wi-Fi, give the service a name, then click Create.Click Wi-Fi.
If the network you expect to see isn’t in the preferred networks list, click the Add button
at the bottom of the list, then click Show Networks. Click the network you want to add to the list, then click Join. If the network is protected by a password, enter it.Drag the the network you want to give highest priority to the top of the preferred networks list. This is the network your computer will try to join first.
To delete a network from the list, select it, then click the Remove button 
.