Download and install Android Studio from here
Open up Unity and go to Edit -> Preferences
In the Unity Preferences go to External Tools and under the Android heading under SDK click browse
Locate your Android SDK folder and select folder. The SDK folder should be located under “C:\Users\User\AppData\Local\Android\Sdk”. You can check where the Android SDK Location is in Android Studio under Configure -> SDK Manager.
Your preferences should look something like this.