My advise centers around excelling at the one thing I see the most complaints about people playing Dark. SURVIVABILITY and TANKING ability. This isn't a guide to Min/max leveling up like the hardcores will. It's focused the first time tank, doing what PF/DF role you fill. Tank.
1) Understand that there is TANK gear and MELEE gear. Leveling up, it's not easy to tell at first if you aren't paying attention to numbers. A lot of gear has STR and VIT on it leveling up and it's easy to think you can wear just anything "Discipline of War" - but that is not the case.
All of your left side gear (Head, Body, Hands, Belt (debateable), legs, feet) Should be for "GLA,MAR,PLD,WAR,DRK" only need ffxiv power leveling. If it is, it's tank gear. The biggest reason I've seen for low level darks being a bitch to heal, is they are wearing DoW gear which has 1/2 the armor.
2) GRIT or Die. While I will sometime drop GRIT when down to a single mob if DPS feel slow and I'm bored, keep this up for 2 or more. Or just leave it up all the time. Focus on being a TANK first. That's your role. I know you want to get all badass and feel like a DPS. Resist the urge.
3) Level up at least GLA to get provoke to start. While I know you want to dive into Dark, by the time you reach 50 Dark, you really should have all your GLA and WAR cross class.
4) IMO, pick 1....a) Spec into STR, have VIT jewerly for dungeons, spec back into VIT at 60. b) Spec into VIT and have STR jewerly for solo, optional VIT jewerly for uber tank survival until 60, then switch to VIT jewerly at 60 full time(this is what I'm doing).
5) Mark targets. You can always do this after the pull if run speed is a concern. I
make my pull, drop an aoe and/or Flash, and immediately mark the target I'm on with 1, give it a few hits, then tab over, hit it a few times, mark 2, flash/aoe again, and tab. Which leads us to...
6) learn to build threat on all targets, marking targets lets your dps not only know what to kill in what order, but lets you weight your time building threat on the lower numbers and rely on Flash/AOE on the mobs that are a few number above the lowest, however ...
7) Go a bit crazy on AOE if you have an AOE heavy party. Manage your MP to prevent having to rest. If your party is FFXIV PS Powerleveling, marking may not be as needed - I sometimes still use it so I can keep track of what mobs I've hit a few times in what order. Unless you plan to burn through MP and need resting/ballad - mixing AOE threat with single target is the most mana efficient and leads to no downtime.
8) The list of enemies that pops up in combat - is a list of enemies you have hate with. Red flashing square means it's on you, Green dot on somebody else (yellow arrows up and down indicate being in between and that you are gaining/losing threat.
9) underneath the job icons, the white bars, are threat meters - when a mob is
targeted, it shows it for that mob. Whoever's bar is full is who that mob is attacking.
10) Be proactive on coodowns. While it's easy to think of defensive CDs as being for when things get ugly - often the better use is when you know something bad is coming - like a boss with a long cast on a tank-buster ability. You wont see this much outside 8-man content - but the point stands, if you know a bunch of ads are spawning you may need FFXIV Company Seals Power leveling, pop that Shadowskin as soon as you have them all rounded up, rather than waiting for your HP to get low. Saving your healer mana is always a good thing. I would encourage you to use CDs even on trash pulls leading to a boss.