Skyrim Special Edition, in spite of the improved graphics, is still the same old Skyrim. Old timers can relive their adventure in a more high-def world, while newbies can now be an adventurer (but don't take an arrow in the knee!) to explore caves, fight dragons, marry off to a lovely lady, or join a guild.
There are four guilds in Skyrim Special Edition and three factions including the faction in the Dawnguard DLC. The guilds have already been covered here, so now we'll talk about the factions.
Skyrim Special Edition Guild Guide: Stormcloaks Faction
Ralof, who you'll encounter in the Skyrim Special Edition tutorial, should invite you to join Stormcloaks. Per VG247, random NPCs will also invite you to get you started. However, you can always go to Windhelm where the faction is located if you really are keen on joining.
Head over to the Palace of the Kings, where Ulfric and Galmar are talking. After their conversation, talk to Ulfric then Galmar, the latter of which will test you. Carry out the quest, which includes killing someone on an island near College of Winterhold.
Return to him and you'll now be allowed to join the Stormcloak faction. There's a goodie for you, too, in the form of a Stormcloak armor. Note that you can't join the Imperial Legion once you choose this side.
Skyrim Special Edition Guild Guide: Imperial Legion
The Skyrim Special Edition Imperial Legion is sort of the Skyrim army led by General Tullius. Very early in the game, someone will invite you to join the faction, but if you miss that character, you can go to Castle Dour in Solitude - their base.
You'll find General Tullius. Talk to him and he'll point you towards Legate Rikke. This will prompt a quest, which you must do so you can join the Imperial Legion. Once you have carried out the quest, go back to Rikke and take the oath.
You'll then be directed to a blacksmith, doing so will award you with the legion's armor. Note that you can't join the Stormcloaks faction once you choose this side.
Skyrim Special Edition Guild Guide: Dawnguard Faction
Dawnguard faction comes from the DLC of the same name and is a group of vampire hunters. So if you're into that, might as well join this one. If you're under level 10, you will need to talk to the Nord Agmaer at Dayspring Canon. If you're at level 10 or higher, a Hold Guard in a hold capital or an Orc named Durak can give you the quest.
The quest will lead you to the aforementioned Dayspring Canon and to Agmaer. He will then lead you to Fort Dawnguard. Just follow the flow and you'll be given a crossbow, after which you will be invited to join their faction. And that's it. You now have access to their quest line and can now play the full contents of the DLC.
Skyrim Special Edition is now available on PlayStation 4, Xbox One and PC.