Service account key

For service account key go to Settings -> Service Account and Generate New Private Key. You'll get a json file.
Replace you service account key with key/serviceAccountKey.json

Use your Project ID

For project ID go to Settings -> General
Replace <Project ID> with your project ID

admin.initializeApp({
    credential : admin.credential.cert(serviceAccount),
    databaseURL: 'https://<Project ID>.firebaseio.com'
});