There are a few steps you need to follow in order to turn on OK Google.
First, you need to make sure that you have the Google app installed on your device. If you don't have the app, you can download it from the Play Store.
Once you have the Google app installed, open the app and tap on the Menu icon (three lines). From the Menu, tap on Settings.
In the Settings menu, scroll down to the "Voice" section and tap on "Voice Match".
Turn on the "Access with Voice Match" option. You will be asked to say "OK Google" three times. Once you have said it three times, the OK Google feature will be turned on.
You can now say "OK Google" followed by a question or a command. For example, you can say "OK Google, what's the weather like today?" or "OK Google, set an alarm for 7am".