Alright, first things first, you need a Firebase project. If you don't have one yet, no worries! Just head over to the Firebase console, click on 'Add project', and follow the prompts. Pop in a project name, agree to the terms, and hit 'Create Project'. Easy peasy.

Next up, let's get your app talking to Firebase. In your Firebase project dashboard, look for the 'Add Firebase to your web app' button. Click it, and a popup with some code will appear. Copy that code and paste it into your app. Boom, you're connected to Firebase.

Now, to use Firebase Cloud Messaging, you need to add a specific library to your app. Open your root-level build.gradle file and add these lines:

dependencies {
    // ...
    compile 'com.google.firebase:firebase-messaging:20.1.0'
}

This will pull in the Firebase Cloud Messaging library for you.

To send messages from your server to users, you'll need a server key. You can generate one from the Firebase console. Just go to Project Settings -> Cloud Messaging, and click on 'Create new server key'. Voilà, you have a new server key.

Alright, now let's get Firebase Cloud Messaging working in your app.

First, add this to your AndroidManifest.xml file:

<service
    android:name=".MyFirebaseMessagingService">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT" />
    </intent-filter>
</service>

Next, create MyFirebaseMessagingService in your app:

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        // Handle the message here.
    }
}

This will let your app receive messages from Firebase Cloud Messaging.

Now for the fun part—sending your first message! In the Firebase console, go to 'Cloud Messaging' and click 'Send your first message'. Enter a title and message, select your app from the list, and click 'Send'. Your app should now receive the message.

Remember, for transactional notifications, target specific users or devices. You can also schedule messages and set up automatic ones based on certain conditions. For detailed information about these options, refer to the Firebase documentation.