How do you optimize your gear?

[LeonKeyh]

Really, while leveling, just equip the highest item level piece of gear for a slot that matches the gear type of the class that you're playing.

From levels 1 to 50:

It's tank equipment if it can only be equipped by the tank classes
It's physical DPS equipment if it can be equipped by "Disciples of War"
It's magical DPS equipment if it can be equipped by "All Classes" (Unless it has Control, Craftmanship, CP, Perception, Gathering, or GP, then it's for Crafters/Gatherers and you shouldn't equip it for combat jobs)

Accessories are all over the place in ARR, but equipping the highest item level with your main stat should suffice.

At level 50 they start separating out the equipment, left hand equipment can only be equipped by the class(es) that they're meant for. Most right hand equipment is separated into:

X of Fending - Tank Classes
X of Slaying - SAM/MNK/DRG
X of Aiming - MCH/BRD/DNC/NIN*
X of Healing - Healers
X of Casting - Magic DPS

Just equip the highest item level of the correct equipment type and you should be gravy.

At 80, you'll want to start melding materia and you may have multiple pieces at the same item level whose stats will matter (slightly), The Balance discord is the best place to determine what is best at that point:

https://thebalanceffxiv.com/

[Rongway]

At 80, you'll want to start melding materia

It's worth melding at least as early as 70. Saturating your gear with tier 6 materia will get you another 680(720 with belt) substat points. And you can freely retrieve tier 6s, so you only need to build a small pool of them to last you for all your jobs.