How to prepare for Final Fantasy XIV: Stormblood
Decide on a job. If you're planning to start one of the new Final Fantasy XIV jobs there's not much to do in game right now. You can keep up to date with news on the upcoming jobs to gain some beforehand knowledge. This will make things a bit easier in the beginning. If you're planning to step into Stormblood with an existing character make sure you've completed Heavensward main story line and your FFXIV weapons has the latest upgrade level.
Get gil. It's no secret that saving the day doesn't earn adventurers much money. If you've only played Final Fantasy XIV combat jobs so far chances are you're still sleeping in the free room at the inn. There are less than 4 months until Final Fantasy XIV Stormblood releases and that's plenty of time to level crafting and gathering jobs for some nice profit. The new expansion comes with new recipes for all crafting disciplines. The new items will be extremely valuable in the beginning. Choose a profession that suits you and start making some FFXIV gil cheap.
Inventory clean up. Final Fantasy XIV Stormblood expands inventory but don't rely on the extra space just yet. Most players have their bags filled with useless items acquired throughout their Eorzean adventure. Potions that might save your life one day, items you're not decided what to do with, a bunch of popotos because you're committed on maxing out Culinarian even though you have touched the skillet in months, all that stuff has to go. Take a day to go through your bags, if you don't know what an item is just look it up on Final Fantasy XIV wiki and get rid of anything that has taken inventory space for months or even years. Before vendoring items, check out the auction house. Maybe you can make a small profit. Don't forget about retainers. Those poor guys probably have their bags full of junk as well.