As in all the chapters of the Kingdom Hearts series there will be an introductory movie to the game, the choice of the difficulty level and a Prologue in which you will learn the basic commands.
In the Prologue of this chapter you will command Ven; Terra and Aqua will be your masters / adversaries to learn the basic techniques.
First get familiar with the Control Pad to move, the O key to jump and the Bumpers to change shot, then use X to attack.
Now we move on to the Cruise which you will need to move in the Command Console menu and the Triangle button to carry out the selected attacks in the Console. When the chest appears, approach and open it to get the FURY ability.
As you advance you will see a new cutscene and take command of Earth. With this new character you will learn to defend yourself with the Square button. Finally you will use Aqua and learn how to use the SHOOT command.
To finish the Prologue you will have to fight using one of the 3 characters. It doesn't matter if you win or lose, the important thing is that you become familiar with the sometimes uncomfortable controls of the PSP.
Now you will have to choose who to start the adventure with, in this guide we will follow the best choice from a narrative point of view, that is Terra, Ven and finally Aqua.
Have fun!