Welcome to the dark side Horde and to the crazy world of RP in particular!
As for guidance finding a nice guild: I’ve always been of the opinion that the best way is to jump out there feet first to do some RP in the wild, see which groups you gel with, seem to be having regular events, attend a few to see if you enjoy them, and then inquire about joining them. It’s not the fastest process, but it is the one that’ll generally result in the least guild hopping. Most guilds I know of are open to people just getting into the Horde RP scene, as well.
Having a firm idea regarding your character and the kind of RP/gameplay and OOC guild environment you’re looking for really helps too.
Here’s a list of many Horde guilds that are looking for new folks!