When you defeat enemies you’ll earn XP and AP. Gaining XP will level up your characters, while AP will be earned by any equipped materia, potentially leveling it up. There’s no need to fret about AP early on - the first two chapters are fairly linear and you’re limited in what you can do, but in Chapter 3 you’ll be able to explore various locations (like Scrap Boulevard) with constantly respawning enemies. If you want, just run around there and grind for AP to level up an orb of materia and pop this trophy… or just play the game, it’s inconceivable that you won’t level up some materia at some point.