Performing at the highest level in WoW is about having access to the right information and being able to act on that information in a timely manner. This is especially true for healers because so much of what healers do is based on the status of their teammates.
So your first goal as a healer should be to have a user interface that lets you make the best choices. The foundation of a good UI includes a few things: unit frames, timers and warnings. There are a number of options for those, but I’ll include my choices to give you a starting point.
First, download Weakauras. There is no other addon in the market with the flexibility and overall usefulness of Weakauras, and there are countless ways that I and every other M+ pusher use Weakauras to give us the information we need. It’s tough to give examples of Weakauras because there are so many ways the addon can be used, so instead I’ll point you to Wago
which is repository for user-submitted Weakauras that is searchable. This site should quickly give you an idea of the breadth of the Weakauras addon and also let you start looking around for specific stuff. If you’re curious, I have my list of Weakauras on Wago
For unit frames, I use the all-in-one addon ElvUI
. It not only includes unit frames but also action bars, chat, bags, interface skins, and more. It can be intimidating to start with, so if you’re looking for a quick start you can import my ElvUI settings here
Everyone needs dungeon warnings, and while you can theoretically tackle all of that with Weakauras, there are very popular addons with dedicated support teams that aim to give you full dungeon support with continual updates. I use BigWigs/LittleWigs, but DBM is also an option here. These addons will show you timers and warnings for important trash and boss abilities.
Another important tool for pushing keys is Method Dungeon Tools. It’s the only place on the internet where you can find the positions of all the mobs and bosses in the dungeon, and it lets you plan routes through each dungeon that can be shared with teammates.
Once your UI is complete enough to give you the information you need, you can start working on your playstyle. Druid healing is all about managing your HoTs -- you’ll never do substantial healing as a Druid unless you have several HoTs up on your target. This is part of the reason why UI is so important for Druids; you need to be able to easily track your HoT uptime on your party so you know when to reapply.
The final piece of Resto gameplay is learning how and when to DPS via Affinità dell'Aggressore Ferino
abilities, aka catweaving. Each group will be a bit different in terms of how well it avoids damage, so your opportunities to DPS will vary from group to group, but you should get a sense for when it’s safe to throw in some catweaving damage because no one is going to take much damage (e.g, Priestess Alun’za).