First off welcome to Roleplaying! I’ve added some helpful links at the end of my reply that I would reccomend reading through as these people have put in a lot of effort for people like you, new to the RP Community.
You dont necessarily need a guild to RP with, public RP hubs like Orgrimmar/Stormwind, Booty Bay and (from time to time) the Crossroads/Lakeshire are filled with lots of RPers that are welcoming “random approach Roleplay”. If it feels a bit intimidating I would recommend finding a guild that welcome new Roleplayers.
A great addon called Total Roleplay (search for TRP 3) is used by the RP Community to make RP-Profiles. This addon is used to show our In-Character name, first-glance descriptions, any character details and any self inserted description. Id recommend downloading this addon to play around with and also look around how Roleplayers set up their profiles.
That’s what I have to tell you, now here are the links:
Newcomer guide
Guilds of AD (not all active guilds are listed here)
Other newcomer having a bunch of different advice given as replies