Hey there! First, good job trying to better yourself and help make running in groups/raids smooth. One thing to keep in mind is that at one point everyone, and I literally mean everyone, didn’t know how to run X dungeon or Y raid. The entire player base learns over time.
If you’re looking for guides I’d recommend WoW Head. Usually they have FatBossTV embedded YouTube videos and I’ve always found those helpful for raids. As for dungeons they usually show screenshots and maps showing the best possible route and which mobs you can skip.
My advice? Start small! What I mean by that is instead of being overwhelmed and trying to learn all dungeons or raid bosses at once, pick one thing and focus on that. When a new raid comes out I usually read and watch videos for the first three. There is no way I could watch 10 videos for 10 bosses without ever stepping foot in the raid, my mind wouldn’t be able to keep it all together. Once I feel comfortable I join a group. My intention is, if needed, to leave after three bosses since that’s all I know but usually people are OK with teaching (more so early on). Also, for raiding you can join “Learning” groups early on or, at this stage, join the Looking For Raid. Even though in LFR no one follows mechanics, you can at least take what you read and see the fight in action before doing a Normal/Heroic raid.
The same goes for Dungeons. Pick one, read about it, then you can join a Heroic version just to solidify what you read with a real experience. Then try a low level Mythic, then once you feel comfortable increase your key. is this slower gear progression than just jumping into a M10 or a Mythic raid? Yes. But your question is about how to get better without causing frustration to a group or to yourself by having people mad at you.
I hope this helps. Enjoy your time in Azeroth!