So let’s start off with the basics. Item tiers go from Normal - Magic - Rare - Legendary. You also have uniques and Mythic uniques in the mix and depending on the build you may need to seek some of these out.
Moving forward though you’ll mostly be concerned with upgrading your gear to Legendary items, switching out the aspects as needed, and looking for the right affixes/stats. I don’t know what build guide you’re following, but I’ll give you a general guide.
For the most part you can’t go wrong with armor pieces that have +main stat and +health. This leaves the third stat on your gear to whatever you might need. Such as reduced cooldown, resistance, armor, Crit chance, +skills or +passives, etc. Typically for your boots you’d want movement speed if possible.
Again this is just a general rule, if you find a pair of gloves with Crit chance, +core skills, and another random stat you want to keep by all means use it. Maybe your chest piece has +all resist, +armor, and a passive skill you want on it. Don’t take this as law, and just go with what works for you at the moment.
Jewelry is more for offensive purposes and some extra passives on an amulet, if you get lucky. If you’re like most of us, finding the perfect amulet will be near impossible for you, so going with something that helps you out for the moment is what you’ll go for.
For example an amulet with movement speed, armor, Health, Crit chance and main stat is a good starting amulet. Combine any of those three on an amulet depending your build and it’ll be useful. Rings you can dabble a little more into damage. Such as Crit chance, damage types, main stat, etc.
Weapons you want to go for something that your build uses. For example if your build uses Overpower, try to get that affix on your weapons, or if it focuses on Crit damage you’d want that, etc.
Once you’re level 60 and have 750 gear (the hard cap before ancestral gear) you can start to add Tempering affixes onto your gear via the blacksmith. You can also upgrade your gear using Masterworking, this will help you push a little farther if you’re struggling a bit, but I wouldn’t waste too many mats on Masterworking 750 gear. I’d wait until you get 800 (ancestral) gear for that.
If you have a link to the build you’re following I can give better detailed instructions if you want.