PS3 noob here with a question on what to focus on....

[Estellios]

If you enjoy PLD play it first. Tanks are in high demand and you can make sizeable amounts of money doing duty Roulettes as a tank as they are almost always the bonus role.

Just make sure to keep your gear, especially your weapon, up-to-date and you should be fine. All the storyline dungeon mechanics are very intuitive to get a grasp of and if you are ever in doubt you can just search online for guides if you feel like you want to be prepared, which I actually would recommend for stuff post-storyline if you are using the Duty Finder. As long as you let people know early on that you are new in a dungeon typically they will be up for offering advice. It's not hard to get into new parties as a tank anyway so if people bail on you you can just requeue.

Another option is to try and use the Party Finder tool to look for members that are willing to go through a learning run through whatever dungeon you are trying to clear. Since you are a tank you would have the hardest role to find filled already.