In fact, Square Enix does a terrible job preparing tanks to tank.
As a tank, you need to keep aggro / hate of all mobs on you, but Square never do tell you how to do that properly.

Eg. did you notice when you target a mob, there are little bars in the team list, underneath each teammember's class/job icon ?
These are the aggro levels of the targetted mob for each player in your team, from 0 (most aggro) to 4 (least aggro) or more later.

Your job as tank is to have aggro level 0 on all mobs.
So you have to cycle through all ennemies to watch these little bars-tards ^^
It'll help you determine on which mob you need to place your next aggro hit, or maybe just Flash them all.

Carefully read your actions effects, and use and abuse of Flash at low levels (but I suppose they told you precisely that already).


In general it's a good start to tell it's your first time in any trial, people can help you and give pointers about particular tank strats, if they're nice.
They can't votekick you out until 5min so don't wait and show 'em what you're made of ^^

Good luck !