Hi, welcome to WOW!
Some things that new players have to know:
The experience of the game is completly changing over time. The first days/weeks you only have to level your char to the max. You can do this by questing or by doing dungeons in the looking for group tool. (as a healer its much easier to get into groups)
A holy priest is a fantastic char, but it has like 20 healing buttons, but only 3 damage buttons. So the damge rotation is really boring and the damage is really low. If you want to level by questing you really have to play as a shadow priest as it makes 10 times more damage and you will get to max level much faster. (you can switch your talents/spec anytime for free). So if you want to kill anything you should always play shadow, only switch to holy if you do group content. In general there is a lack of healers, so normally you have a very good chance to find groups. But holy priest is the less desired healer as is has the fewest support abilities. But its playable anyway. (only if you want to play with the top 0,1% of the players (M+ 18 etc.) they will maybe not want you .
Once you reached max level the game is completly changing what suprises many players. Lets say there is an optimum what is possible if you play your char absolutly perfect. For questing its enough to play 10% of the optimum. You can almost use randomly any buttons and the enemies will die. While questing you make a lot of progress: new levels, new skills, new items etc. (In the endgame you will not get any new levels or skills and after ca. 4 weeks you will only get maybe 1-2 items per week, so the progress is slowing down)
Later with max level you will start with normal 5 man dungeons and ca. 20 man raids, there you need to play maybe 20% of the optimum. Then HC dungeons where you need 30%, then M0 where you need 50%. Reaching M10 you need 90% and so on.
The Game is changing, at the start you moslty play alone, but almost all endgame content is group content, for 5 or ca. 20 players. So almost no more questing, just farming gear in groups in harder and harder getting content.
So if you have started right now, just try to get max level as fast as possible, as the real game starts with level 80. Everything before 80 ist only the tutorial. Before 80 just try all skills and all talents and keep what feels good for you.
As a Healing addon i just use “Grid2” for 20 years now and i do often play 80-95% logs.
Later you could read a guide, but for now try to stack Int and maybe crit. But item level is the most important thing. Below level 80 you don’t have to heal that much, many dungeons can be cleared almost without any healing. Real all skills and try them. Look for big numbers heals and for synergies between skills.
BTW Thank you