Getting to Ishgard

[Enkidoh]

As Elysaine mentioned, you need to have completed the entire '2.X' series storyline to enter Ishgard (and thus play the expansion content), as the storyline leads to there. The 2.X story consists of the original ARR story that ends at the quest The Ultimate Weapon, and which then was continued through five separate patch storylines - 2.1 A Realm Awoken, 2.2 Through the Maelstrom, 2.3 Defenders of Eorzea, 2.4 Dreams of Ice, and finally 2.5 Before the Fall.

Each storyline continues where the previous one left off, making it one very long continuous story arc. As Elysaine pointed out, the very final one that the Heavensward story then picks up from (which grants access to Ishgard as a part of it) is the patch 2.55 story quest Before the Dawn. It will probably take a couple of weeks to get through all this story (and it's connected dungeons and trials that you have to do as a part of it).

Don't despair though! Take your time going through it and enjoy the story at your pace (in other words, don't let players boss you into 'skipping the cutscenes', particularly with Castrum Meridanium and the Praetorium.) If you're curious as to what you have to expect, this is a list of all the current story quests in the game: http://ffxiv.gamerescape.com/wiki/Ca...Scenario_Quest (it should go without saying but there are possible spoilers there).