How do I make my MacBook Air stay only on my WIFI?

Please read macOS Sierra: Choose preferred Wi-Fi networks.

If that does not accomplish what you seek consider the following.

Click the Wi-Fi icon and select Open Network Preferences...

Select Wi-Fi in the left column, then Advanced...

Under the Preferred Networks list, select every Wi-Fi network that appears, including the one(s) you want to use, and delete every one using the "—" (minus) button. Select "Remember networks this computer has joined" and "Require administrator authorization to ... change networks" if they are not already selected, assuming those options describe your intentions.

OK then Apply.

Select the wireless network you want to join in your Mac's Wi-Fi menu, and provide its wireless network password. That network will be the only one it will join, until you manually select a different one.

Some articles are to be followed : macOS Sierra: Connect to the Internet using Wi-Fi

: macOS Sierra: If you don’t see Wi-Fi in Network preferences

: macOS Sierra: Set advanced Wi-Fi options

Make sure you add Google DNS server settings as 8.8.8.8 and 8.8.4.4 also location must be automatic , no proxies must be enabled .

Open System Preferences > Network then select Advanced then select the Wi-Fi tab.

Drag the preferred Wi-Fi network to the top of the list then click: OK