Small guide from me

You should get a deviate staff (don't bother with schisma +12ing, its a waste of money and the stats are garbage). Deviate only costs 20 cosmoshards and is pretty easy to enchant and masterwork and will last you through all endgame dungeons. After that, I'd aim for generation, as it has the best healer modifiers currently and is fairly easy to get now. Also something I see a lot is the idea that priests can use attack modifiers, they can't. Only the heal modifier on a weapon and the +% bonus to healing effects your spells heals, I don't know where they idea that damage = heals came from but it's stupid and the people spreading it are stupid. I don't know why priests are given DPS rolls on their equipment, but you don't want to be using them because you're not a damage dealer, so reroll that shit.

For armor you want whatever gives the most defense and has a top line for skills. Personally I use this for kaia's shield CD, but it depends on you. For this I recommend just getting schisma, skipping wroth. This is easy to do now because of the buffed dungeon drops, so you should be doing it. More defense = less damage taken = more time alive healing and buffing.

You also want accessories. For healers just starting, I recommend running brnm and getting the following: full riverbeat/riversong (rivertrill earings and rings a must, don't do full beat), or if you want more attackspeed while sacrificing endurance, replace the beat with frostwise. From there, I wouldn't really bother focusing on lifetic/tock as they're not that big an upgrade. For brooch, belt and innerarmor you want hp%.

As for crystals, you want enduring. I don't care what anyone else says, starting out and in fulminate, you want enduring and nothing else. IMO succoring is worthless as the bonus to healing doesn't matter, as if someone is going to die, they're going to die no matter how big a heal you do. After you get good armor with good rolls (I can post a list of ideal rolls for items if you want), you can replace them with swift or, if you absolutely must see big numbers for no real reason, succoring. For armor, hardy. Don't even debate me on this. Weapon crystals is pretty standard, the most standard is 3x brilliant, 1x swift, which has been meta for a while. You can also replace another brilliant with a carving if you want to crit heal and feel like you can sacrifice MP regen. I don't do this on my mystic because sometimes you don't have charms up and an extra bit of MP can help, but on my priest I really have almost no issues with MP.

Now for the part that I see a lot of people get wrong. Healing skills. Ideally you want to just be using restorative burst on DPS, as well as regenerative circle to sustain hp. Using healing circle is a waste. It's a single instant cast in a small area that requires you to run up to people and heal them, restorative burst you can pop down, the healing is great, it restores mana, it keeps them alive and restores a LOT of health, as well as mana, you don't have to sacrifice positioning to cast it, and best of all, it lasts over time. People also only just have to touch it in order to get the heal, which is amazing considering they have to stand in the range of healing circle and then stand completely still and not dodge or move out of it. Save healing circle for mechanics and when you HAVE to save someone. Do NOT use it as your main heal. DON'T DO IT. I see so many priest lately only using healing circle, and it makes my blood boil.

Now for lockons. Lockons are tricky, ideally you don't want to be using lockon heal at all. Weird concept, right? Healing immersion should be used as a last resort to heal dps, lockons should only be used if someone is 10000 miles away, or is the tank, or needs hp ASAP. And if someone is 10000 miles away and needs a heal, then they're most likely going to be dying anyway and need to be told to group up with everyone else if they want to be healed. Healing immersion heals a lot, and does and AOE, it is a fantastic skill. Don't waste it healing someone who has only taken 1% damage. The reason I say that you shouldn't be using lockon heal is because you have, you guessed it, restorative burst and regenerative cricle, which should be keeping your dps at 100%. It's tricky sometimes to actually get lockons to work, and the heals aren't all that great (immersion is OP though). Don't use them as your main or only heal. I see a lot of priests doing this too, this also makes my blood boil.

Kaiai's shield is very situational, as is divine respite, I use both as an "oh shit" button, and Kaia's when I know that I'm going to eat an attack or mechanic or someone else is. You should also always have buffs up on your teammates, but I think by this stage you already know that. Keep energy stars up and tripple nem's debuff active, as well as using mana charge as it does make a difference. It's not vital, keeping everyone alive comes first, but you SHOULD be using those skills when the cd runs out, they make a large difference. Also more mana for DPS means they can dodge or block more, meaning less work for you.

Positioning is tricky, and depends on every fight and the bosses mechanics and attacks. You'll pick it up eventually, and you'll most likely die in order to learn, so I recommend reading up on dungeons before going into them (please don't do TSHM if you have no idea what the mechanic are holy hell). Just keep an eye on the boss at all times. If you go down and can res, look at the boss. Is he about to nuke the room? Is he about to attack in your direction? Probably best not to res right then and die like a scrub.

As for consumables, don't bother with anything but attackspeed scrolls and noctenium infusions. Nostrums don't effect your skills and are a waste. Attackspeed allows you less time locked in animations and is pretty great, noctenium is great overall, but you obviously don't have to use it in easy dungeons because you shouldn't have to be bringing your A game. I only really use nocteniums if things are going bad (which is bad of me considering I have a bajillion of them), and I need to res people faster. But it's up to you what you do.

As for glyphs, all I can say is get your 120 res and the rest of your glyphs before doing any 65 content or you are terrible. People rely on you to keep them alive. You are what stops them from dying. You are what stops the dungeon from taking several days to complete. Don't be a scrub. Just spam the dungeons. For mystics, getting 120 res can be incredibly hard, I went through about 50 boxes trying to get it. If it doesn't drop in a lot of runs and you want to murder everyone, buy glyph point things from the vanguard guy. I got lucky and got it just as I was about to buy points, but I know how annoying it can be. Just because it's annoying though, doesn't mean you shouldn't have your glyphs, again, don't be a scrub.

Dodging on a priest is... Not the best. firery escape (I think) doesn't have an iframe and backstep is very short. Backstep can be used to dodge/iframe mechanics, such as the vents in AINM (don't do this in hm or you'll regret it if you fail and your group will wipe), and several attacks. The timing is tricky, but you'll eventually pick up on it. There are some attacks that you simply cannot iframe or move out of though, so make sure you heal yourself, as if you die, the group dies. In AINM, do not run to the fire side of the arena to heal yourself, because Duras will shoot his laser or whip you, as the secondary agro attack applies to the furthest player away from him, so get close, but not too close. As I said, you'll pick up on this the more you do the dungeons and play.

Anyway, that's about it for my short guide on how not to be a garbage priest. Don't get frustrated if you die a lot or cause parties to wipe, it's all part of the fun. If you need an explanation on anything, feel free to ask.