Main WHM here since 1,5 Years.
Some small tips that will help you be a good whitemage:

- Whitemage is a GREAT healer. Purest healer in the game, straight and simple. Also, the only job in the game, that gives a Mind party bonus. And...The healer with the strongest AOE heals.
- Focus Target. It was explained already how it works. ALWAYS use that, for EVERY boss (not needed on dungeon trash pulls etc~). You NEED to see what the enemy is doing, when he is charging a big attack, so you can react in time. So you PREcast a heal, that goes out imediately after that attack goes out. If Tank takes 10 000 Damage, and then you're like "oh damn i should start healing him", it might be too late.
This will also help you to understand the boss behaviour better.
- Regen. That is your by Far best single target heal spell. Total potency of about 1100+ or something like that. Keep it up 24/7 on Maintank, during the Fight (do not precast it on any pulls)
- Cure > Cure 2. If you can get away with only spamming Cure 1, prefer that. Cure 2 is only your emergency heal, if your Cure 1 per GCD is not enough. That way you get the most Freecure procs, which allows you to use your Cure 2 without MP cost --> MP efficient.
- Divine Seal. Learn to love it. Free 30% stronger heals on a 60s CD. Use it a lot for big heals, or just to save MP (to make your cure 1 stronger, and not need Cure 2 anymore)
- Shroud of Saint. Learn it, Use it. It has 2 major uses: 1. Your to-go MP regeneration skill. Learn to time it properly. Not when you're out of MP, but when you hit like 70-80% MP. On lvl 50 it restores 1100 MP, on lvl 60 it should be around 4500~?!). 2. Your current Enmity (aggro) will be reduced by 50%. It will just "take" half your enmity away. Usefull for a WHM, becasuse you are AOE healing alot sometimes, and on fights where the boss jumps away for 3 minutes..... Tank can't generate more aggro, but you do with your heals. You could get aggro at some point, and this helps. Which leads me to......
- Keep an Eye on your Aggro bar. If the boss turns Red, you have Aggro and he will come to smack your face. Pop Shroud of saint fast enough. Even if that shouldnt happen a lot, its possible. Doesnt hurt, if you have a good overview.
- AOE Healing: Medica 2 > All. Use it, and let the HoT (Healing over Time) get people back up (if there is no other AOE attack from Boss in the next 20-30 Seconds of course).
- Learn when to prefer Medica over Cure 3. When you are with a good group, or a static, for a lot of AOE attacks the group will stack together. No reason to use Medica then, chose a smart Target (like yourself if you are in the middle), and use Cure 3. Especialy if it crits, you will almost jizz your pants, when everyone is back to full from barely surviving. If Random groups, they might be spread out, and your Cure 3 would hit only 3 people (It has a 6y Radius. Medica has 15, Medica 2 has 20)

- Dont let ever any WHM tell you, that Crit is a Bad thing. Its not. It never was. Imagine Tank droping from 11k HP down to 2k, because of a big Hit from a boss. And you time Cure 2 well, and it crits for 6k, instead just healing 4k. Would you be sad, like "oh fuck not a crit again, i hate it"? No you won't. You love it. At least i love every single crit my WHM heals.
What makes too many people hate it is "unpredictable overheal". Like, tank has 1k HP missing, your cure 1 heals 1k, and it crits. 500 HP overheal. But well, fuck it. Its 500 HP free healer. In THIS case it wont help you. In many others it will. You dont throw a Cure just ebcause a tank got pinched for 400 damage.
This does not mean, you should go all for Crit. No, just that Crit is not bad. In Fact, i even prefered it over Spellspeed untill now, because spell speed was, in my eyes, useless for a WHM. Your heals are situational, and timed. And never "spammed"..
DET is considered the best for WHM, because it gives you more consistency in your heals, and less "RNG". But now, Crit doesnt only make you crot more often, it makes your crit harder. So, crit is better than ever. Even for WHM.
Overheal: "Overhealed" HP generate double Enmity than "missing" HP healed. Which leads to increased enmity generation = you might grab aggro.
Therse a simple solution for that: Stop playing with scrub tanks. A good tank wont lose aggro even CLOSE, even if you have 60% overheal. Which doesnt mean, you should overheal. You WILL get a feeling for when to heal smarter, and more efficient. Like, maybe wait for the Tank to lose 2k HP, instead 1k to start your heal, on some trash. Wait a half second more, see if you get a Cure 2 proc, etc etc. With that, any occasional crit won't lead to overheal. But like i said, you just have to keep playing WHM and you will get your own feeling for that.
But anyway... Since theres pretty limited gear choices now, that leads me to the next point, stats:

- Use your 35 points on Mind. All the way. ONLY exception: VIT, if you do Endgame Raid content progression, and need a larger HP Pool to survive certain Attacks (due to beeing slightly undergeared at the beginning)
- Gear stat (2nd Stats only. Mind is obv #1): Piety > rest. Really. Piety is the best stat for any progression. You dont know the content, you might heal less efficient, because you want to keep everyone up, in case something happens. More Piety = More MP = More natural MP Regen per Tick (2% of your Max MP per 3 second). Anything else... Det, SS, or Crit... really... dont give a fuck. If you happen to have 2 pieces of gear with the same item level, you might think about it. But for Whitemage is simple: Higher Item Level = Better gear. So just increase your item level as much as possible (Weapon > Rest), and don't think too much about 2nd stat. They matter for Whitemage the least, in my opinion.