Professions in Shadowlands will be significantly different than how they worked in Battle for Azeroth. This page provides an overview of the new items and professions changes in Shadowlands.
Crafting your own Legendary Armor is one of the new features in the Shadowlands expansion. This guide tells you more about the whole process and explains what you need to do in order to craft your first Legendary item.
The Shadowlands pre-expansion Patch 9.0.1 is going live on October 13th in NA (October 14th in EU)! There are a lot of changes to classes and basically everything, but there are very few profession changes. I will only focus on the new Relic of the Past optional reagents in this post, but you can read through all the new changes on mmo-champion.
Optional reagents are new types of reagents that can change the item level and the secondary stats of the items you craft. You can probably guess from its name, but it isn't required to fill the Optional Reagent slot when you create something, you can craft an item with no optional reagents or you can fill as many Optional Reagents slots as you prefer.
Patch 8.3 Visions of N'Zoth goes live January 14th in NA (January 15th in EU). There are no major changes in this new patch. We will basically get higher ilvl version of the 8.2 recipes. There are no new materials for these recipes, they will require ores/herbs from Nazjatar. (the mobs in the new assaults drop 8.2 leathers and cloths)
I recommend Zygor's Leveling Guide. The guide is in a small window on your main screen in-game. All the detail that you need to complete the quest is included. You’ll never have to switch back to a website, or even the in-game main map to see where you have to go or what’s next. The guide is available for both Horde and Alliance.
I use Zygor's guide for leveling and I highly recommend it.