advice for a first time tank?

[NyarukoW]

If level-synced moron insist on grabbing aggro from you, let them eat the mob they get. Do not mass pull and speed run without coordinating with your party.

[Ryock]

Starting with MRD/WAR will make the learning significantly easier imo. Mostly because MRD/WAR don't lose hate anywhere near as badly as GLD/PLD do when you're a fresh learner. You will be queued up with players with significantly greater gear than you, so you will struggle to hold hate on any enemies whatsoever, and it'll throw off learning the game since you'll be spending the majority of the time just running after the monster while the Bard kites it for an hour. Once you've learned the essentials on MRD/WAR, you can go to GLD/PLD if you prefer that play style. It's my own personal suggestion.

Other Tips: Marking enemies as 1, 2, 3, 4, etc will give a kill order to your group. Hotkey these so you can give the group a kill order to keep the fights structured and orderly. COMBO YOUR SKILLS. Too often I see PLDs and WARs just spam Butcher's Block or Rage of Halone. This is a waste of time. Always use your combos. You are better off using Shield Lob or Tomahawk as they have more damage anyways, and thus give more hate. I can think of plenty of other tips, but most people in here have covered the important stuff. It can be stressful as people expect you to lead the group and tank perfectly. You can be blamed quite a bit for things that are sometimes out of your control. Keep a positive outlook and you'll be fine.

[Koenignovi]

Trash Mobs :
Hate control is your main concern. AoEs work well for holding groups but when MP or TP is low, know when to swap hate tactics. Don't be afraid to rotate chains on groups of trash.
Don't start out Speed running unless insisted upon and if you know the strat for the speed run. You can learn and enjoy the SR experience, but not everyone is for it.
If someone insists on pulling trash before you, let them know you they do at their own death. Provoke is nice, but it only works on ONE mob every 30sec...

Bosses :
Learn the fights. Even if you have too on the fly.
Pay attention to the battle field and the mechanics of the fight, things can change rather fast and you're the epicenter of it all.
Communicate, be vocal on your experiences and short comings. After all, this is a game and we all need to learn some time. Everyone has their own way to learn.

The Role :
Be prepared for criticisms. Everyone has their two cents.
Listen and observe. If you feel what you are doing is right, don't waiver.
Be willing to die.
DPS is not your number one concern, but it does make a difference in the long run. If your group is out DPSing you by huge margins, you will need some help picking that up.
And most importantly, HAVE FUN! Don't let it get to you when things go wrong. We play this game as a release, not as a job.

[SpookyGhost]

Close your eyes and believe.

[Neuflune]

Close your eyes and believe.

Best advice.

Tanking is hard especially when most DPS is at endgame ('coz everyone wants to DPS obviously).

Social Advice: Work with party members who want to help you, those who have some tips about tanking. Don't get easily offended by "I main as tank so do this and that"; the rest of what they say may not be as 'rude' as you think.

Practical Advice: Some of this would really depend on your tanking class. WAR has Overpower which is better at grabbing and holding aggro, but it consumes TP. PLD is great for single-targerts (especially bosses) and all of those defense buffs really help a lot. Always keep aggro on you, check the aggro bar on the party list. Number targets, even if people don't follow them at least the fault's not on you. Dodge AoEs. Even things that don't have AoE markers (such as the dragon's Frost Breath in Stone Vigil, Lion's Breath by that other boss that has Swinge) can be dodged if you move as soon as you see the skill brought up. Don't forget to click off Regen. Don't forget your buffs (Defiance, Oaths). Learn how the skill Provoke actually works. Oh, and Stoneskin when you can.

[Luvbunny]

Yeah I don't really get how people are so dumb when it comes to "speed run". When your healer refuse to heal you, you are not "speeding" the run, you are slowing everybody down instead of just doing steady pull, which is lightyears faster since no one is dead. Unless you go with pre made group who knows what they are doing and veteran on "pull everything and the whole kitchen sink".

[Illya]

When you're just starting out, simply make sure everything is hitting you and not your party, and try to not be dead. It will be hard to die in the early going if your healer isn't AFK so focus on learning how to draw and hold mob aggro. Hell, a SCH can pretty much put you on follow and AFK and let Eos heal you the whole dungeon and nothing will happen early on.

The rest you'll figure out as you level your way to 50. The game does a good job of teaching tanking mechanics to new tanks, unlike many other MMOs.

[Bluevann]

Be modest, I tend to start every instance (even those I've done a lot) with "Hello, please bear with me if I lost hate. I'm still trying to learn/my gear isn't the best/I'm rusty/It's been a while/etc." People tend to mellow out at that and be a lot less nasty if you do lose aggro sometimes (and they also try less hard to get hate, which I always appreciate as a tank).

Use markers. If people don't follow them, ask once or twice that they please do so. If they get snarky/snappy, ignore them and just stop asking. If they get aggro it's their fault at that point.

Don't leave roulette instances just because you don't like them, soon as you enter. Stay for at least 15 minutes and give that party a go; sometimes the right party can make an ugly instance enjoyable. And at that point, if you really want to leave at least it's been long enough for the others to hit Abandon instead of all leaving/causing others to waste their queues by entering an empty duty.

Level both tank classes, you'll need crossclass skills from either.

And last but not least, don't let people get to you. Tanks have somewhat a reputation of being "spoiled pricks", and people feel they have the right to treat them as such without thinking that not all tanks can be spoiled pricks. The party can't go on without you (unless they're in a under-30 instance and they have an arcanist along or a DPS with tank crossclass skills); know this and try be the adult of the situation at all times. Giving in to flamed arguments will only end up being a waste of everyone's time.