Hiya \o
Tanking can be quite easy in dungeons it's all about holding Aggro, well in any content really lol but that is the main job of tanks.

In dungeons, Def CD's aint really a must to have, you can go through out all the dungeon and not pop a single one unless the group wants you to do a big pull if you do end up doing a big pull just make sure your geared for it ok and don't pop all your CD's at once! The most easiest job to pick up tanking in is PLD, Physical Defence wise he's quite sturdy, simple and easy to play and has quite a few good CD's.

Holding Aggro is they key thing for a Tank to do, for AOE PLD's Have Flash, WAR has Overpower and DRK has unleash. Spaming these once or even twice during pulls should help keep the aggro on you but if you notice you have a BLM or SMN who like to do AOE damage best to throw in a few more Flash/Overpower/Unleash etc.

Marking, If you mark the mobs killer order it makes the whole run got a whole lot easier as the DPS know which mob to focus on and for you to keep aggro on.

Put your Bonus stats into STR or VIT. Some people will say put your bonus stats into VIT but I prefer STR, reason beining is that the extra STR will help you hold Aggro more easier and if your a WAR can restore more HP with your bloodbath/ innerbeast etc attacks.

That's the basic of it if you choose a Tank Class I can give you some more tips hints on it.