Welcome to AD and to our hobby of RPing 
I hope you’ll find lots of friends and make memories that lasts like so many of us already have done!
To answer your questions:
Addons: TRP3 is an amazing addon that lets you customize your characters profile, add backgrounds, what people might see when they get their initial impression of your character etc. It’s a great tool, while there are some other addons that does the same thing, TRP3 is the most commonly used one.
Location: The capital cities of each faction usually has a lot of RP going on outside of some areas usually reserved for OOC (out of character) actions. Seeing you are Alliance, here are some common areas I know of for your faction
- Stormwind, everywhere except the Harbor and Trade District.
- Stromgarde, Arathi Highlands gives you a great alternative to find some RP in.
- Boralus, has seen some play, and most notably has the beginner RP friendly tavern every so often. [A-RP] Beginner Bar RP - The Curious Octopus
Character Backstory: While having some idea of your general backstory is great to have in order to base your characters reactions towards, it’s not necessarily needed. I start my characters off with very little, building slowly upon their backstory as I get to know my character as well! I would, however, read up on basic Dracthyr lore. https://warcraft.wiki.gg/wiki/Dracthyr
Guilds: You’ll find a guild through interacting with people, that’s how I find mine. However, if you wish to learn more about guilds on your pass-time you can check out their forum posts here on AD-Forums or on the site ArgentArchives. https://www.argentarchives.org/
Community: There are many active discord communities here and about. While I’m not in any Alliance communities I’m sure someone could send you an invite to the Stormwind, Stromgarde, Boralus discords.
If you have any questions, do reach out. Most often than not you’ll find supportive help within our community! Have fun in the world (even if you chose the wrong faction) and be respectful to others