First of all, go to the Google Api Console and select your project 
And select the api you want to include, for example, Google Map Api or google place api. Turn on the api.

Add api key in manifest application tag, e.g.
<meta-data android:name="com.google.android.gms.version" android:value="@integer/google_play_services_version" /> <meta-data android:name="com.google.android.maps.v2.API_KEY" android:value="@string/map_api_key" />
if you use both Map and Geo Api. Then add geo api (it will work for both)
<meta-data android:name="com.google.android.geo.API_KEY" android:value="@string/place_api_key" />
source share