Welcome to Eorzea! Tanking is an artform that does take some getting used to, and especially early on. Luckily though, GLA is actually quite easy to learn to tank with, as the abilities it gets to do this are very straightforward in how they're used. The first thing I have to ask though, is have you done your level 15 class quest from the GLA's Guild in Ul'dah? If you have, then you would have gained an ability called Shield Lob. This ability is very useful for tanking, specifically, 'pulling' the enemy ('mob') right at the start.
Once the enemy has reached you, use Flash when it's close. Flash, despite it's name, is very similar to FFXI's Provoke - it is your main hate generating tool, along with your weapon skills. Then keep alternating between Flash, and the weapon skill combo Fast Blade-Savage Blade or Fast Blade-Riot Blade (the former generates more hate in itself, but the later one restores MP, which is very helpful if you're using Flash a lot, because Flash is a spell).
After three or four rounds of Flash/Weapon Skills, the enemy should be well and truly focused on you (your character should have the letter A over their name in the party list), if not already dead. Then simply rinse and repeat on the next enemy. Your buffs like Convalescence and Rampart are also useful, but they're better saved for a tough enemy like a boss.
The ACN you had in your party might have also just been trolling your party sadly - most know not to use a tank pet in dungeons unless absolutely necessary for that reason - so don't take it to heart.
As for the posting restrictions on the forum, a lot of that was because of trolling on the Beta forum got really bad and thus had to have similar restrictions added, and I assume SE just didn't want to take any chances with the proper forum as well. They might remove them in the future, but the restrictions aren't too hard to get around (the 1000 character limit is anyway - just post your first paragraph, then immediately go back and edit it.).
Anyway, hopefully this has helped - asking for help here is already the first step done, as we all had to learn the game sometime (some of us are still learning).