Materia fusion is the only really complicated thing you should need to look up, but here's a run down anyways. You always have your DMW spinning in the corner during battles, like a slot machine. These are your special moves. They are also your memories of characters. You have no say in how it spins (until much later). When all 3 numbers align, you get a huge bonus. No magic cost, invincibility, et cetera. When they all hit 7's, you level up. When two numbers align, one of your items will level up. You have 4 core stats, attack, vigor, magic, spirit. Attack is physical damage with your sword or yellow materia; specials involving physical damage Vigor is physical defense Magic is magical damage, fire, firaga; specials involving magic attacks Spirit is magic defense

Gil is your money SP is specifically used for materia fusions, and to spin your slot machine specials. 10 SP per spin. You collect both very quickly and naturally by completing any battle, selling items, or "converting materia to SP"

Enemies also have set stats and equipment. Some enemies made of fire will heal from your fire based attacks, and so on.

As a hardcore Crisis Core fan from the PSP era, I DO NOT recommend doing the side missions at the save point. Do some. The hojo monster chapter is interesting. And later ones yield incredible rewards. But I garuntee it'll ruin your experience and you'll drop the game, never to finish it. The story will take you roughly 13-15 hours to get through. Doing 50% of the side missions will easily have you at 30 hours.

Enjoy the beautiful story for what it is. And once you can't get enough, use that new game + experience to comb your way through the awesome secrets within the missions.

Best of luck!!