Admob

Procedure: Interstitial and banner ad.

1. Create a new project or open your existing Android project in Android Studio 
Then ad below line of code in build.gradle inside your dependencies and sync project before proceeding further

compile 'com.google.android.gms:play-services-ads:8.4.0'

2. Now open your activity layout and add below lines of code. In my case I am pasting it in activity_main.xml file.

<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="@string/banner_ad_unit_id">
</com.google.android.gms.ads.AdView>

3. Open MainActivity.java file  and add below two lines to your main class.

InterstitialAd mInterstitialAd;
private InterstitialAd interstitial;

4. Now add below lines to your onCreate method in MainActivity.java file.

AdView mAdView = (AdView) findViewById(R.id.adView);
AdRequest adRequest = new AdRequest.Builder().build();
mAdView.loadAd(adRequest);

// Prepare the Interstitial Ad
interstitial = new InterstitialAd(MainActivity.this);
// Insert the Ad Unit ID
interstitial.setAdUnitId(getString(R.string.admob_interstitial_id));

interstitial.loadAd(adRequest);
// Prepare an Interstitial Ad Listener
interstitial.setAdListener(new AdListener() {
    public void onAdLoaded() {
        // Call displayInterstitial() function
        displayInterstitial();
    }
});

5. Add below method to your MainActivity.java file. If you don’t know exactly where to add code just paste above last curly bracket in the code or watch video embedded above.

public void displayInterstitial() {
// If Ads are loaded, show Interstitial else show nothing.
if (interstitial.isLoaded()) {
interstitial.show();
}
}

6. Now add below to lines to your strings.xml 

<string name="banner_ad_unit_id">ca-app-pub-3940256099942544/6300978111</string>
<string name="admob_interstitial_id">ca-app-pub-3940256099942544/1033173712</string>