For new players there are plenty of resources including:
(1) extensive in-game guides on how to play the game
(2) new realms for new players
(3) countless free third-party resources and sites like wowhead, icyveins, method
(4) guilds that new players can run with
(5) proving ground
I am ALWAYS understanding of new players as at one time I was one myself, but people have their limits and if the tank is going to mess around instead of doing their job, I am going to (A) pull for them, or (B) vote to kick them.
Keep in mind this happens VERY unfrequently as the vast majority of the dungeons I have been in have gone smoothly.