Flag the relic quest and do as much as you can when you can. You'll proceed with it as you gear up and gain access to more things. The i80 (i90 with a little soldiery invested) weapon at the end of it is a good start.

Then unlock things. Mor Dhona will be awash in quest markers. Most of these will be dungeons, most of which you won't have immediate access to, but you'll want to do them when you can. Additionally you'll want to unlock the CT set of 24-man raids as soon as you can. These raids have low item level entry requirements and award gear that's useful to you right now.

Your most important task is to get gear. Do all your available roulettes every day and buy the i100 soldiery gear to rapidly increase your item level.

It can seem overwhelming at first, but just flag everything you can and do them as they come up.

Also proceed with the main story. You'll need an average item level of 90 to enter all the duties required, but it ramps up slowly. Heavensward is a little over a month away, but you definitely have time to finish the 2.55 story and be ready for it.

Welcome to endgame.