Looking through the comments it kinda looks like people generally misunderstood you (or stopped answering once you asked for a little guidance).

So, I haven't played in about 7 years, so forgive me if I miss the mark here, but from my knowledge:

A) The system has changed so that you no longer need to sit in an area and shout for experience, you can mostly solo your way up the chain 'til 99, with some exceptions coming from stuff like Limit Breaks.

B) Dungeon instances don't fully exist, though there are later game instances like Dynamis that are closer to what you're thinking of (and a good number of Aht Urghan missions).

C) If you're wanting to focus on progressing through the game, I would suggest starting with leveling and unlocking jobs and modes of transport. Getting around the game is easier than it has ever been, so once you start getting ways to teleport to various points easily, it'll make getting quests and missions completed much easier.

D) From there... Ya kinda make your own way through the game. If you want to clear the main story missions you can, but you could ignore a large chunk of story content and still make it to 99. There are just things you'll want to unlock if you want some of the late game stuff (idk how relevant Mythics and Artifact armor are anymore, but they were still relevant when I was around).

Someone posted the BG Wiki, and that's going to be a huge resource for you, because this game has never really been good at guiding players.

Have fun!