The most important component of phaser may very well be the main phaser game object constructor. This is what I call to make an instance of the main Phaser.game object that I typically set to a variable called game. This main game object is then referenced a lot, in each state of the game. Long story short this is where the magic happens with phaser.
First off it might be best to start by reading my post on getting started, and the state manager in general with phaser. Phaser is a very complex project, but is well worth the investment of time to study it. In this post I am covering how I go about getting a basic loading screen working okay involving a progress bar, and more than one State object.
When getting started with phaser it is important to have a certain foundational understanding of some of the core features of how phaser works, no matter what the project may be you are going to want to know a thing or two about State Objects.
If you are new to phaser you might want to start with my getting started post, and also there is the StateManager, and the Main Phaser.Game constructor that is of interest also.
When I start making a javaScript project they almost always involve some kind of main game loop, and in more advanced projects an array of objects that contain methods for each game state. In Phaser there is the StateManager, and State objects that are of concern when it comes to getting this done with phaser, rather than my usual vanilla js solutions.