Once you have the equipment you like, you can start upgrading it. These are quite rare, and I've only really seen them from chaos chests, which now randomly drop while doing high level dungeons, and be acquired with patron (5 a day) or via the store. Equipment only has 3 stats, so to get a 4th stat on an item you need to use a Pearl of Wisdom. The important thing to note is that you can reroll the 3rd and 4th stats with a Tentacle of Q'bthulhu at a chaos forge, so if the top two stats are what you need, the rest can be rerolled to what you need later on. For that, I used this spreadsheet, which is not mine and I take zero credit for it. Once you're to the point where shadow items can drop (seems like uber 3 or 4ish), you should start looking for shadow equipment that has good stats for your class. So relic items can't be upgraded to resplendent quality, and as a result you should only use stuff below that until you have shadow stuff. Items below this can't be upgraded to a new tier.

Black items are shadow, and these are the ones you should concern yourself with.

Trove has your typical rarity system, with blue, green, orange, red etc items. *** Spoiler - click to reveal *** Your weapon, face and hat all add PR.

Trove how to farm golden souls