Speaking of that... how do I know when I have crossed over in to the Heavensward content? I got the expansion free here recently when Square was giving it away, but i know nothing about it personally.
Will it be a natural progression of the main story line from the original game in to Heavensward, or will it be like the stupid shit they do in games like EverQuest 2 where you have to track down some npc to start that content?
It just chains. If you catch up, it'll stop and when a new patch is released, you have to figure out where the NPC was when you last turned in a quest, but as long as you're doing content, it basically chains one after the other without interuption. If at any point you're lost, there's the Scenario Guide thing in the top left that tells you where to go next(as well as your class quests location, do these too, they give gear AND skills). But since each expansion simply continues the MSQ before it, there's no looking around for a NPC to start the new arc, it literally just keeps going, expansions aren't self contained, they're all part of the main story, just different arcs of it.
As for Heavensward, you'll know when you're in HW content because the very first quest is literally "get to fucking Ishgard bro", so once you go to Ishgard, that's when Heavensward starts. Well technically it's when you talk to the NPC right before that, but same thing.
Also all the quest sounds(taking or turning in quests) are different based on the expansion. And after doing hundreds of ARR quests, you'll very obviously notice when you pick up the first HW quest and it has a fancy new sound fanfare and on screen effect.
Mind you, if you're 25, you're REALLY far from HW. Take your time and all but yeah, you shouldn't expect to get there soon, this game has a shit ton of story, it's like 25-30hours for the ARR MSQ and then another 15-20 for the post ARR leading to HW. Then HW and SB are basically about 20hours each for the main MSQ and 12-15h for the post MSQ in each. And that doesn't even scratch the amount of side content.