This is in the Chain of Memories section, so I'm going to assume that you're playing CoM. (And if you aren't talking about CoM, then this is in the wrong section anyways) In CoM, you can only play as Riku after you have finished Sora's story; so after you reach the credits, while playing Sora, then you return to the menu screen, it should let you start a new game with Riku.
As for fighting Maleficent, attack the flames she spits at you, and you'll get a green trick card; that gives you the platform to make it easier to attack her. Besides that, you should just make sure you have plenty of good cards in your deck for sleights, combos and stuff; I hear using Cloud sleights is good against her.