OS X Yosemite: Choose preferred wireless networks
By default, your Wi-Fi connection attempts to join the wireless network your Mac was most recently connected to.
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.
OS X keeps track of the wireless networks your computer has connected to, and lists them in the Wi-Fi pane of Network preferences.
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Choose Apple menu > System Preferences, then click Network.
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Choose Wi-Fi in the network connection services list, then click Advanced.
If Wi-Fi isn’t in the list, make sure you have an AirPort Card in your Mac, then click Add
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 networks you expect to see aren’t in the preferred networks list, click Add
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.
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Drag the name of the network to the top of the list to give it the highest priority. This is the network your computer will try to join first.
To delete a network from the list, select it, then click Remove 
.