Only some people can use it. You have to have the potential, first. And you use one of these.

[He holds up his Evoker.]

A Persona is like... a manifestation of your personality. It's a mask you wear to face the world. People who summon them use them to fight Shadows. There's a lot of different kinds of Persona, but most people can only use one. I do know of a few people who can use multiple ones, though.