You will get introduced to all this through the campaign, so don’t worry if you don’t understand all this stuff yet.
Renown is a character power system tied to covenants - which for you is Night Fae. By doing campaign quests and Callings you get Renown - which increases the standing with your covenant and thus gives you better world quest rewards and most important: unlocks Soulbind Conduit slots.
Early in the Covenant Campaign you get introduced to your first Soulbind Niya and you get your first conduits to use in the Forge of Bonds. Higher renown unlocks more Conduit slots and Soulbinds, and thus your character power. Conduits is rewarded from World Quests and Callings at this point.
During the Campaign you will also be told to go back to Oribos and return to the Maw to rescue Thrall, Baine, Jaina and Anduin. It is during this quest line you unlock Torghast and the Runeforger.
The Runeforger is the guy that crafts your Legendary, but here is the cumbersome part:
In order to craft a specific Legendary, you need a Memory of the specific Legendary you want. Here’s an article covering the available Legendaries for your spec and how to obtain their memories: https://www.wowhead.com/guides/elemental-shaman-best-legendaries-shadowlands
When you have obtained the Memory, you have to use it in the room with the Runeforger, so he can learn how to craft that particurlar Legendary.
And no, we aren’t done yet. Now we need the following reagents:
Soul Ash and Soul Cinders, which are rewarded from completing Torghast, Tower of the Damned. There are two scenarios on daily lockout, so you can do the two available scenarios daily until you have the Soul Ash and Cinders you need.
2x Missives, these are crafted by Scribes and buyable on Auction House. There are four different Missives: Missive of Haste, Missive of Critical Strike, Missive of Versatility, and Missive of Mastery. You can’t use two of the same in the Legendary, you have to pick two different missives.
1x Base Item. This is crafted by gear crafters for all gear slots, meaning Leatherworkers for your armour type (Mail) or Jewelcrafters for the neck/ring slot. All buyable on Action House. The Base Item for Mail Armour is called Boneshatter, and you will find it on AH for many slots and a wide range of item levels. The Base Items for neck/ring slots are called Shadowghast. The higher the level the base item have, the more Soul Ash/Cinders is required to craft the Legendary.