Hey. o/

True WHM healing gameplay IMO doesn't begin until level 52 where you get Secret of the Lily. These are 3 gauge resources that go up over time in combat - one per 20 seconds - and give instant healing. I don't think you get much to do with them outside of Afflatus Solace, which is a single-target instant heal. You can get rid of Cure I completely, Medica II sees uses and IMO will continue to until you get your AoE Lily heal (Afflatus Rapture), Cure III is a MP-hungry AoE that you can use Thin Air on to make it cost nothing but honestly in my opinion Medica II will be the better substitute.

On how to heal dungeons, I follow certain tips. I put the tank on focus, try to learn all the defensive icons to know if they'll be in danger or not. If they slip to 40% or low, slap them with a heal. If the situation is tight and I see no defensives on them, at 10% or so I hit them with Benediction, a spell that tops them off to max. All the while, you can do damage. Medica II early on can be used to heal up for group-wide damage, but do replace that with your AoE Lily heal once you get it.

WHM healing is very "GCD" focused - know how when you use certain abilities, some go through a brief timer and some are still usable? WHM's damage abilities - Aero/Dia, Holy and Stone/Glare trigger the GCD, and so do the instant Lily heals. There will be some outlier abilities that you can still use while the timer is going on - things like Benediction, Tetragrammation and other spells at later levels. In the White Mage Job Guide or tool-tips, if they are on GCD they will be shown to have a recast of 2.5s, and if they are not they'll show as Instant cast and a longer cooldown recast. So hypothetically, if you see the tank go low on health you can use Stone on your enemy, and then hit him with Benediction right when the cast ends (as Stone's cast is shorter than the GCD - 1.5 second cast, 2.5 second recast). Other healers can do more direct healing when all the while doing damage. Being able to do a GCD ability and then using an "off-GCD" is called weaving.

It sounds complex, but this is really how healers have to continue doing both damage and healing at the same time. WHM facilitates it relatively lesser, but you're not completely helpless. I played Holy/Discipline Priest in WoW in 9.0, and the gameplay there is different because it's somewhat impossible to do this sort of global cooldown weaving given the shorter time.

Remember your ABC - Always Be Casting.

Also, I would suggest holding off on reading full-length guides until like, level 60-70. The sheer girth of spells may overwhelm you, so progressively going through your Job and learning that way would make you feel less uncomfy. Many Jobs tend to really expand and even transform in gameplay when they reach Stormblood.

Hope I helped.