build.gradle(モジュール)

...
dependencies{
    ...
    implementation 'com.google.android.gms:play-services-ads:19.6.0'
    ...
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest/>
    <application>
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>
    </application>
</manifest>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout>
    <com.google.android.gms.ads.AdView
        android:id="@+id/adView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_alignParentBottom="true"
        ads:adSize="BANNER"
        ads:adUnitId="ca-app-pub-3940256099942544/6300978111"/>
</RelativeLayout>

MainActivity.kt

class MainActivity : AppCompatActivity(){
    override fun onCreate(savedInstanceState:Bundle?){
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        //追加
        MobileAds.initialize(this)
        val adView = findViewById<AdView>(R.id.adView)
        adView.loadAd(AdRequest.Builder().build())
    }
}