Trying to hold aggro as a tank at low levels is complete cancer. Your group needs to wait 2-3 GCDs for you to establish preliminary aggro or else they're going to rip it every time (if they're good DPS or bad healers). It's not your fault, it's the game's for not giving you the tools you need until later. But here's some tips:
Gladiator: Not so bad pre-30. Always start your pull with Shield Lob (if single target) or body pull + Flash (if multi-target). Fast Blade -> Savage Blade will allow you to keep aggro on a single target without eating too much TP. For multi-target, spam Flash, and use Fast Blade -> Riot Blade to regen MP when you're good on hate. You can use Provoke to rip aggro from someone if crap hits the fan, but keep in mind that Provoke only gives you 1 aggro over whoever has the most hate on a target, so never use it to pull and always follow it up with a Shield Lob, Flash, or combo'd Savage Blade. Level 30-39 (when you first become Paladin), you might as well not even bother trying to tank anything. Without Shield Oath (obtained at level 40) and the increased aggro generation it gives, you'll pretty much be helpless to keep aggro off of DPS unless you're spamming Flash like a madman, because while all the other classes got an increase in power, you didn't get an increase in your ability to hold aggro. 40+, tanking is an absolute breeze.
Marauder: An absolute nightmare pre-30. First thing you want to do before even playing Marauder is level Gladiator to 10 so you get Flash and Convalescence, these are two abilities that are absolutely required for tanking as a low-level Marauder. (You'll eventually want to level Gladiator to 22 for Provoke, but you won't need that until endgame.) Always start your pull with Overpower (preferred) or Flash (if you need to save TP), regardless if the pull is single-target or multi-target. Keep aggro on single targets by using Heavy Swing -> Skull Sunder, or multi-targets by spamming Overpower/Flash and praying that the mobs die before you run out of TP/MP. (With a full MP bar, you can only use Flash 4 times before you run out of MP, no matter what level you are.) Bloodbath can be used as a cooldown to generate more aggro because healing generates aggro, but Berserk is going to be your go-to for generating aggro. While Berserk is active, you're generating 50% more aggro, even with Flash. The downside is that for 5 seconds after it ends, you can't use any weapon skills (but you CAN still use Flash). Once you hit 30 and get Defiance (from WAR), aggro management becomes a breeze (even easier for multi-target when you get Steel Cyclone at 45).
Universal tip: If the Conjurer or White Mage is constantly ripping aggro from you, tell them to stop spamming Medica.
