As far as I know, there's no addons for FF14 nor allowed. I'm coming from WoW too.... Well I still play WoW, just don't care for it that much. Play it enough to keep myself ahead between Heroic / Mythic geared.
As for how to heal, the best way, is trial and error. If you're shy about it, grab a friend and duo queue some Trials and Alliance Raids. Ask about what they start for rotations. In a nutshell and in general for WHM:
- Keep Medica II up when you know consistent damage is coming. (I recast it when there's 5 seconds left on the hot)
- Use Divine Benison on the tank when you know big damage is coming, to help mitigate it
- Cure III is extremely niche and specific situations to heal with it. Only use it when the group is stacking and needed
- Cure II is one of the main heals you'll do consistently on tanks or to top off folks
- Cure I use for minor damage and / or to get the free cast for Cure II
White Mage in a nutshell, there's more to it learning and what to do for reacting but I hope that leads you somewhere.