Other advice I can give you is to start working on endgame as soon as you can. Now, I'm not telling you to farm like crazy on low level mobs for gold to buy level 60+ gear, but bear in mind what certificates you want to use and what armor you want to aim for endgame. Riding on free quest armor is what I suggest until level 50-60 where it starts becoming worth it to invest in crafted stuff.
If you see rings or necklaces drop with 2-3 types of elemental resist, keep them! As soon as you hit level 60, you start being bombarded with pretty high power boss AoE even as healer, and bosses will sometimes use attacks that hit a random target regardless of aggro. The cool thing about elemental resists on accessories is that the amount you get from a level 23 ring is the same flat 8% bonus that you would see on a level 63 ring, etc. Bear in mind, these are only a temporary emergency measure, though. Eventually, you want to complete some Legendary Achievements for crafting to boost your base elemental resists and collect some certificates for it, but until you do, swapping rings around on any class can help as a handicap, especially if bosses keep wiping the healer/dps with strong elemental AoE. However, it's understandable that one might have limited bag space early in the game, so try not to keep ones with overlapping effects or with too few resistances. Also, there is a necklace from Avila Volcano (Fame chest) that gives 8% Fire resist and 5% of all other elemental resists. This one is handy since it A) gives more Fire resist than any other necklace B) is very versatile and C) can be archived and taken out anytime to conserve bag space. If you have inventory problems and can't afford to expand your bag, only keep the best of rings and capes (be picky by requiring 3 elemental resists).
And for Legendary Achievements, if you have gold for it, try to complete the ones most pertinent to your class choice, but balance with cost and level. For example, crafting necklaces might give one of the stronger effects (flat 3% bonus to Atk Speed and Cast Speed for the first two levels and 4% for the third) but costs a lot. So maybe craft the listed level 1-30 necklaces and wait until you have more gold to finish up. Starting with all the basic achievements such as 1-30 of each armor and 1-30 for maces is really easy (actually, collecting the 1-30 mining materials, summoning materials, and farming materials might be even easier) so it's good to start there. However, the 1-50 achievements start to become quite daunting and wallet draining, so pick carefully which ones you do and at what level you do it (gold comes easier at levels 45, 51, 55, 59, and 62 where new dungeons one can spam solo become available). When you decide to shift towards M-Crit healing, make sure to do ring crafting for 5% Crit Dmg/M-Crit Dmg each level.
Also plan your certificates. You might need to branch out a bit to get all the certificates you need, such as playing Mage to unlock Illusionist and Warlock, in turn playing those two classes to obtain Cast Speed and Crit Dmg certificates. Shaman is particularly important (and particularly irritating) to level since you get elemental resist, M-Crit Dmg, and Cast Speed certificates from leveling it. My best suggestion is to try to sneak into parties as Shaman and heal for them, because leveling Shaman by beating mobs . . . it's almost painful unless you have really strong magic. And even then it's slow at best. Bard and Mage give M-Crit Rate, which you can use early or later since you might want to dabble in dealing damage as Cleric when you reach level 40, or if you feel Holy Smite is too slow to cast practically at that level, about level 60. Cleric using Holy Smite is a high damage, stunlocking threat that can be used in both PvE and PvP. I'll let you see what it looks like on your own instead of trying to describe battle Cleric since words are hard to do it justice. However, this is totally up to you since if you're a dedicated healer, leveling up in parties is fine too (though please pamper your Shaman starting early on; you'll need it leveled to 60 minimum for all the certificates you need). Note that Templar gives elemental resist from its level 20 and level 50 certificates. It's up to you whether you feel comfortable leveling Templar, but if you train both Knight and Cleric, you will unlock it easily and Crack Shot is the suggested trait for Templar anyway since it is heavily magic based. One more reason to level many classes is for Class Expertise. For each level of Class Expertise you reach, important stats like HP/MP, P-Atk/M-Atk, and G-Healing receive a passive boost. This is the game's way of rewarding players who take advantage of the class switching mechanic to the fullest.
I'm sure you've read this somewhere, but I just wanted to give one more final warning about M-Crit healing. True crit healing NOT something to be taken lightly, and it isn't simply switching from Wis/G-Heal to M-Crit Rate/M-Crit Dmg. It's like keeping most of your strong base heal from Wis, but becoming slightly more reliant on G-Heal, then overreaching your previous boundaries and stretching your abilities to encompass M-Crit Rate as well. In other words, it takes a lot of this: $$$$$$$$ and time to gather certificates, trophies, gear, etc.
What to do now: Considering which classes to train and work on basic Legendary Achievements 1-30. Maybe collect resist accessories.
What to do later: Finish Legendary Achievements slowly but steadily, actually train classes for certificates, and find a way to make lots of gold.