Why do they try to make tanking harder?

[Raymeo]

They're the ones with the problem, not you. That is... unless the DPS that keeps pulling aggro is able to tank just fine, which might actually be the case if you're in a level 16 dungeon. If nobody's dying, then nobody will blame you if you just kick back and go with it.

Otherwise...

let them die

Pretty much this.

Of course first, if they are causing the party to die and it's not a simple accident, you should point out the problem and ask them nicely to stop. Assuming they don't stop though, then your solution is to simply stop enabling their counterproductive play. Let them die. One of three things will happen: (1) Ideally, they'll get the point and quit the act. Thank them for fixing the problem and resume enjoyable play. (2) They'll rage quit. No skin off your back. Worth it to wait a few minutes for a replacement instead of banging your head against a wall for an hour. (3) Unlikely worst-case scenario, they don't learn, and they don't rage quit. Instead they stubbornly and deliberately continue acting as they were, and they do so just to spite you. In this case you simply vote-kick them for griefing. You've asked them nicely to stop, and the party saw the whole thing. You'll get the votes you need. Again, worth it to wait a few minutes for a replacement instead of enabling a jerk and crippling your fun for an hour.

Lower level tanking is a pain, but thankfully the lower level dungeons are also very forgiving. Mistakes shouldn't be too dangerous until you get to Brayflox's Longstop, but I promise that holding threat gets A LOT easier by then as you start unlocking better abilities. Until then just stick with it and do the best that you can. The dungeons that you're seeing now are basically training grounds -- mistakes are okay here, so don't sweat them for now. Focus instead on building up your groove and practicing good habits (positioning, communication, monitoring threat on multiple targets, paying attention to enemy cast bars). Somebody mentioned marking kill orders -- this is something else that you should practice now, because it's definitely a good habit to learn, but don't expect your markers to be followed just yet. The other players in these early dungeons are still feeling out their roles as well, and again -- these first few dungeons are very forgiving, so other people might not see the need for such focus. Don't let them get to you. Practice for yourself. As for basic ability rotation, you typically can't go wrong if you pull with Shield Lob (ranged GLD quest reward), Flash the pack of mobs once or twice (so healers don'to pull hate before you can secure all targets), and then proceed to rotate through mobs, giving them each a full threat combo one at a time until they're all dead. (Of course adjustments will be needed from time to time as you're forced to react to things -- including your own party's actions, not just mobs -- but such adjustments will be easy to figure out once you're practiced at monitoring threat levels.)

By the time you get to Longstop these habits will all be natural to you, you will have likely unlocked Paladin for extra defense, the difficulty of mobs will start forcing your party members to play well and obey your markers, and you'll have been awarded your first Oath ability -- it's Sword Oath, which is a stance that gives you extra damage, but don't discard it as useless. The extra damage still acts as a passive threat bonus to all of your actions, and will make a noticeable difference until you get the even better Shield Oath. You will notice a huge difference in threat as you unlock both of these, and over time you also get stronger finishing moves for threat combos, the pinnacle being Rage of Halone. By the time you've got Rage of Halone and Shield Oath, you'll be an unstoppable threat machine. You'll be tanking with your eyes closed and one hand behind your back, and you'll look back and laugh at the frustration you felt in the lower levels. You'll see.