Heya Loony,
I suggest looking at The Horde Guild Directory(Seeks Guilds!). This will help you get a better understanding of WrA’s guild culture and who to find. Big guilds like Life, Epoch, and Trivial Pursuit are some of the big time raiders on Horde side.
I’m unable to gauge what you’re looking for in RP, so I would suggest starting with more social guilds, like Firebrand and Gigavolt. Valley of Honor is also ripe with many other guilds that are too numerous to list.
Alternatively, if you’re looking for a mix of both, I suggest taking a look at The Vol’kar Legion. We combine a healthy mix of RP, PvP, and PvE. We have pushed and achieved every AoTC since Archimonde (WoD) (sans KJ because he’s dumb) and we’ve tried placing our big toe(s) into mythic but have never successfully done it.
Welcome to WrA! Hope you enjoy it here.