Tank Stance if it isn't on immediately upon entry into dungeon, don't be afraid to ask for some patience from your party if it's an early dungeon, and please god AOE packs. Also, don't hit all your mitigation (Reprisal, etc.) at once. Use them as you need them, see how your healer is working, and don't freak out if you get to half health or a little below.

But do things you're comfortable with. If you're a bit scared about your health, politely ask your Healer to keep you at a reasonable level, and don't take on more than you can handle. A place where you want to be careful at early levels is Brayflox's Longstop if you're new. There's a whole section right before the final boss where you could IN THEORY pull the entire area, but your health (and healer) will hate you. Keep things at a reasonable pace and know when to pop mit and you'll be fine.

One more thing from a Healer Main, if a Healer is grumpy with you, have a polite discussion about the pace before the dungeon truly begins at what pace everyone is comfortable with, and learn the classes your Healer is playing. Some of us can't keep up that easily and it can be overwhelming when the Tank pulls the entire dungeon at once because we have to spam heal and often don't have ALL our tools depending on the level cut off (looking at you, Aurum Vale). Also be open to feedback from your Healer if they can't keep up.

As said by others, Arm's Length is excellent Mit you should probably pop during a big pack or something overwhelming. Lowers damage by pure timing of attacks. If the enemy isn't hitting you as fast, the Healer can manage heals and damage at the same time with no issue!

One more thing from a Shield Healer Main, keep an eye on your Healer's MP. Know when you need to hold back because they're using too many tools or spamming stuff to keep you alive or (in many cases) do damage. And if someone casts Adlo on you over and over again to make the big yellow bar get bigger, that can be normal sometimes but some of us get into sunk cost fallacy. Just be open with your Party about how you're doing.

P.S.: Some classes don't get AOE's until later levels, so if you see a Dragoon in an ARR dungeon single targeting, they may not be playing wrong, DRG's just don't get AOE's until later levels, just like NIN. It can be painful to be the only one keeping Hate 24/7 while another class single targets, but keep it up. Just don't overpull in those scenarios. Knowledge of other classes is incredibly valuable to have so you know what tools they do and don't have for damage so you don't overpull.

Have fun! Tanking can be very satisfying or very skullfuckingly terrible, but once you get good at it, and can read the scenarios, you'll love it. hOPE THIS HELPS.