A lot of different questions from a new player...

[ErnestoGiovanni]

Look at this picture too ther green text tells you what level you can equip it and right above that it says what level the item's stats are.

[Anova]

For your benefit, the armor posted by Ernesto above is an armor for lv.24 or higher Gladiators/Paladins, Marauders/Warriors, and Lancers/Dragoons. However, its item level is 26, so it is comparable to a lv.26 armor for those classes.

Also, be aware that some pieces of heavy armor take up 2 armor slots (usually body+head or leg+feet). You have to add the two pieces together to get an accurate measurement of how much armor you are actually gaining/losing when you decide whether or not to wear it. Many players wrongly assume that their heavy iron armor is the same as a mythril breastplate, although that is only true if they are not wearing any headgear at all.

[Zilon]

thanks for all the great replies....I appreciate the help.

Another question. In dungeon, there is a bar that slowly fill up at the top left of the screen. I am not in the front of the game right now and I dont remember the name of it. I seem like something that build up and then you get something positive out of it. Can someone explain it to me?

[Enkidoh]

That would be the Limit Break gauge - it's basically a special ability anyone in the party can use - the exact effect of which is dependent on what class role the person who uses it currently is (a damage dealing class does a powerful physical attack, a healer heals the party, a tank buffs the defense of everyone, and mages do a powerful magic attack).

Naturally the more filled the gauge is when it's executed, the more potent the effect, hence most players like to 'save' the Limit Break for the end of the fight to 'wrap it all up', and usually will prefer a mage (like BLM or SMN) to use it only, as the attack can be extremely powerful (at full strength the mage LB casts Meteor that does enormous damage to all enemies). That's basically it in a nutshell.

[Niwashi]

Another question. In dungeon, there is a bar that slowly fill up at the top left of the screen. I am not in the front of the game right now and I dont remember the name of it. I seem like something that build up and then you get something positive out of it. Can someone explain it to me?

I think you're thinking of the Limit Break guage, and yes, once it builds up you can get a massive skill from it. It's party-wide, so anyone from your party using a Limit Break will knock the gauge back down, so there needs to be some consideration to who uses their "Limit Break" action. Melee DPS get a massive single-target attack, and that's a common favorite for finishing off the final boss of a dungeon. Caster DPS classes get a big AoE attack that's good if a lot of adds join the boss. Healers and Bards get a big healing limit break that heals the whole party. Tanks get a limit break that significantly boosts everyone's defense for a while.

Here's a page that explains it in a bit more detail:
http://www.ffxivinfo.com/content/limit-breaks.php