Tank Club is a great resource and easy to understand, dedicated to tanking. There is more to tanking than knowing mechanics and skills (which may differ depending on content) and sets (which may differ depending on group), you also have to become a good tank. Learn to think like a tank, learn the details like how to keep up your sustain. This takes a long time, I've been tanking (not exclusively) for about a year now with people able to help me and I still have a ways to go.
Also, tanking dungeons and tanking trials are very, very different. They're practically different roles, honestly.

Learning to tank is "trial by fire". Groups will fail, but you will become better over time. I'm not a great DK tank but I can tank vet HM dungeons on my sorc. I run my ranged taunt (inner fire), immobilize, and vatashran sword and board (void bash) on my backbar to group trash and DSA sword and board (masters) on my frontbar. For difficult bosses without much trash or for HMs I switch my backbar to ice staff with a 1-bar set (ex. yoln) so I can use a monster set. I keep my chain on my frontbar since once everything's grouped, I stay on frontbar except for buff reasons.

My rotation for trash is to switch to backbar, range taunt the big adds or throw caltrops if there are none, run in and activate void bash then roll dodge through the trash (to turn them around for dps flanking), immobilize, caltrops, puncture as many remaining adds as I can, starting with the big ones again, chaining any stragglers and immobilizing again if needed.

For bosses, I run in and range taunt before the boss notices me (despite seeing you first, it may go after a groupmate) and roll dodge through again to turn the boss around. Then puncture and buff up.

There are some other things to keep in mind.
One is Line of Sight (LoS). If an enemy can't see you, they'll move until they can. You can use this to your advantage to move enemies that aren't chainable.
Another is that many bosses have cone attacks. You want to stand still or move predictably with the boss facing you, with your back by a wall to limit the amount of ground the cone covers.
Additionally, sustain. You do not need to block every light attack. This is something I still struggle with. You WILL have to heavy attack since you're a DK (as sorc I use dark deal instead). Since you're a DK your sustain comes from the helping hands passive, from poison abilities, and from using your ultimate. Helping hands can be activated by spamming igneous shield or using something like ash cloud. Poison abilities include things like leeching plate, which is a great 5pc selfish set to get started with as it provides you a LOT of healing. I believe DKs also run charged poison glyph on their frontbar 1-handed weapon, other tanks largely run decisive instead. Finally, watch out for orbs from groupmates, they help a lot and give sustain through block (you do not regenerate stamina while blocking, unless you're on an ice staff with trifocus).
You want to reach maximum resistances while buffed, which the DSA sword and board helps with. I don't recall the exact number off the top of my head.

A TLDR on gear is that I run Naz/Turning Tide/DSA frontbar/Yoln backbar (ice, infused crusher) for a boss setup, Turning Tide/Yoln/DSA frontbar/Void Bash backbar for trash. For getting started though I highly recommend Leeching instead of Yoln. If you ever adventure into Necro tanking, don't use Turning Tide since you have colossus, don't use Naz either because you can't Naz your own colo.

Attributes, all health. Tripot potions. All sturdy, heavy gear. I'm sure I'm missing things but this is a lot already. Hope it helps!