hello!! i hope you're having a good day too ♥
honestly this probably sounds like very obvious advice lmao, but the best way to make an oc is ALWAYS to be as self-indulgent as you possibly can. take some shit you like and run wild with it! whether that's making an oc whose backstory slots into some of your favorite in game lore, a character archetype you really like...... or like, idk. someone who's just really fuckin attractive according to your tastes lmfao. that's my favorite shortcut on how to have the most fun.
as for rp.... one thing i've learned over the last year or so of learning to rp myself is that if you're upfront about being new or anxious, people are really kind and welcoming! don't be afraid to tell rp partners you're inexperienced or to ask questions, bc folks will almost always be thrilled to accommodate you. we're all a bunch of losers playing barbie dolls together, don't let yourself get psyched out too hard LOL.
that being said, if you want more concrete advice about etiquette, commonly used terms, etc: i read this guide (https://tinyurl.com/FFXIVRPFAQ) cover to cover while i was working up the courage to attend in-game rp events and it taught me a LOT about the basics that i was too embarrassed to ask my more experienced friends about lmao. highly recommended!!