This is a list of resources you can use to improve your preservation gameplay, some of them are more advanced than others and some require way more effort to understand but all of them contain usefull information that you can apply to be a better player
Wyrmrest Temple is currently the main hub for high level Preservation theorycrafting and gameplay, there's a lot of high level players active at all times of day to answer questions and help you with any issue you might have. Most of the people that maintain the guides and run the numbers on everything preservation related are active in there.
Maintained by Voulk, QE is the best source for general healer information and hosts the QELive tool for healer simulations. Here you will find articles with high level knowledge about upcoming content and QELive will be your main tool when you need help picking between pieces of gear or choosing great vault rewards.