In Riders of Icarus, honestly I don't think there are any real guides yet, however there are some tips that you can get from this sub-reddit and general information searching on the net and some interesting videos on YouTube. I know as a priest, at least at the initial levels, I've felt very powerful compared to while I had my assassin at the same lower levels (don't depend on this later on though). Here are a few tips I've learned since starting to play - I know there are many people out there much more advanced in the game than I, but hopefully these will show you a little help.
You should make sure do all of the quests – including main and side quests, you may even choose to do some repeatable quests now and again. Last thing really you want to do is fall behind in level if you can avoid it. Even being a 1 or 2 level deficit can be a major pain with the amount you'll potentially miss, I'm not saying you can't deal with mobs and quests that are a level or two ahead of you, it's just easier to keep at level.
Tame familiars as soon as the quest presents itself and if you want to get taming rewards (press B, there are collection rewards shown in the bestiary for the different areas) and many of the ones you need for the first collection are back at the crash site (where you first will end up after the initial "getting familiar" set of cut scenes and basic game play portion). Taming can be made easier if you beat a potential familiar down a bit.
This is much more so with elites and heroics vs commons, but it's good to practice on ones that won't kill you easily to get the hang of weakening the potential familiar then attempting to tame it (this was based on a tip I read and responded to on here - at first I had no idea I could actually attack and still tame, I thought I had to have someone else weaken the potential familiar first). If you get kicked off, even if you didn't attack it before, the potential familiar will attack - even so you can attempt to tame again, you'll just take some hits along the way. If you release a familiar you specifically tamed for the collection (like one that really kinda sucks otherwise) it won't count against you, the collection still shows that you have successfully tamed that familiar so don't keep familiars that you won't want to spend the time leveling up for other purposes unless you want to sink a bunch of Ellun's into expanding your familiar storage.
Gather whatever you see along the way (copper, silver, gold, mushrooms, sage, iron, etc.) - you don't need to go to a trainer for gathering and if you plan to do any crafting at all, might as well gather while you're getting started - the newbie area usually doesn't have a ton of competition for nodes and they re-spawn fast.
Tempering is basically enhancing, for weapons it will increase the damage it does, for armor it will increase the physical and magical defense.Temper weapons as convenient - during leveling while you still have somewhat crappy gear you don't necessarily need to temper to max, but improving the gear a bit can really help.
Also, make sure you level up the familiar BEFORE you seal, once sealed, it won't level anymore and you're stuck with the stats that it has at that point. The higher level the familiar the better numbers it will offer when sealed. Once you get to the point where you're sealing familiars to "enchant" your gear, make sure you read what the familiar will do BEFORE you pull the trigger to seal - no point in wasting a familiar with stats that won't help you.
Crafting is still somewhat hit or miss at this stage, the basic recipes you'll make to level up are not outstanding, but you can find some interesting recipes that you may like. You aren't limited in what you can craft - you can do it all if you want. I tend to do it all so I have it in case I need or want to use it. I've found the most helpful for me has been alchemy (potions), barder (pet related equipment and training gear) and cooking (who doesn't like food). Mostly because I can actually use the basic stuff until I could find / buy better recipes. But like I said, I do it all because I never know what I might come across that I want to make and better to have it leveled and not need it than need it and not have it leveled.
Make sure you read the tooltips - some of the skills are designed to be chain cast to hit harder/stronger.
As a priest you will be constantly generating and expending motes. You can have a max (not sure if this ever increases later, but I doubt it) of 3 motes up waiting to be used. At the beginning you get a few skills to expend motes, one (Sacred Impact) does a nice amount of damage in a small area hitting up to 5 mobs. Some skills also are designed to be charged up. You can use them without charging, but you get a lesser effect (your first ally healing spell is one that charges)
Well, so far I've only had to use the tempering stones. In terms of more costly, if you're trying to temper higher level gear - i.e.: one that can be tempered to 10 or 15 (max tempering according to the in game tips is 20, not sure if that's really max or not, but that would only be on some high end equipment) and you're attempting to go as far as you can, you might find yourself running out of feed stock weapons and resorting to getting them off the auction house (although I've bought a piece of gear or two off the AH, it wasn't to use as feeder stock). The extractors do cost more at later levels since you need higher grade extractors to extract from higher level weapons.
The level 10 extractors are good for (kinda self-explanatory) up to level 10 gear, level 20 up to level 20, etc. The cost does go up a good amount per increment but I've not had anything at an un-affordable level yet based on silver/gold on hand. I've only had once piece of gear so far not allow me to extract from it with the appropriate level extractor - I'm not sure why off hand yet as I didn't see anything in its description that made it clear that it couldn't be extracted from. There may be a version purchasable by Elluns, but I've never looked for them on the store before.
In terms of keeping some extractors on hand, at least while leveling through these low levels, I'd probably just get them in town as you need them. You might find yourself with equipment level 11+ and end up with a bunch of useless level 10 extractors if you buy too many.
Right now don't worry too much about the cost - as long as you don't go insane on the AH buying (unless you are one of those masters of the market that buys and sells well) you should have more than enough Riders of Icarus Gold to buy the appropriate extractors when needed.
Hope the above tips can give you guys some help.