Greetings, and welcome to RP!
I would recommend rolling alts on the RP servers that interest you first, and checking out the community to make sure it’s a good fit. Not every server is created alike, after all. The three biggest RP servers are Wyrmrest Accord, Moon Guard, and Emerald Dream(though this one is PvP oriented and suffered a bit recently). Smaller servers like Cenarion Circle don’t have the sheer numbers for RP, but may be more to your liking if you prefer tighter-knit communities.
If you transfer your character you will be needing to find a new guild to join on your new server. However, with the introduction of communities in BFA, it’s much easier to keep in touch with friends! Additionally, some RPers run RP communities, so it’s possible you may not need to transfer at all–dig up a few communities that look interesting, and you can join the events via cross-realm sharding.
It depends on the guild, really. Regardless of whether or not an RP profile is required for entry, I would highly recommend downloading anyway and filling it out. It’ll allow others to see more information about your character that may not be portrayed by the game model, and will also allow you to tell who’s in-character(IC) and who’s out-of-character(OOC). And vice versa.
TRP 3, MRP, and XRP are the main add-ons for this, and function mostly the same. I prefer TRP 3 myself(due to the sheer customization it offers), though it can be a bit daunting starting out. MRP is a lighter weight version of TRP 3, and the add-on I started with. The good news is all RP add-ons now work cross-server, so characters do not have to be on the same server to see each other’s profiles. Oh, and don’t worry–regardless of which one you end up choosing, you will still be able to read profiles from other add-ons.
You don’t have to know everything about the lore, but it helps to do a little reading beforehand to be familiar with the history of what you’re trying to RP. The in-game lore is pretty good but can be confusing at times, and there are things that happen in the lore books that we…don’t really ever see in-game. Additionally, some races will require a little more lore knowledge to RP well than others(like night elves, for instance, due to the sheer amount of history behind them).
However, it’s also OK to make mistakes, and all of us have done so–even the veteran RPers. If you have more lore questions but aren’t really sure where to start researching, asking here in World’s End Tavern is a good starting point.
As stated before, Wyrmrest Accord, Moon Guard, and Emerald Dream are going to be the big three to choose from.
WRA has the biggest community for Horde RP, but is split about 50/50 overall on faction population(so there’s still plenty of RP on Alliance too!)
Moon Guard is heavily weighted towards the Alliance, but does have a small, closely knit Horde community. It can be a little more fast and loose with the lore than WRA. Just…stay out of Goldshire.
Emerald Dream is the go-to for RP-PvP, although I’ve heard it’s been suffering from an influx of PvPers who do not respect the RP portion of the server. I’m not really sure what the status of it is now–I think it’s been recovering. But if you want to combine PvP and RP this is probably the go-to server for it.
Smaller servers like Cenarion Circle don’t have the numbers of the previous three mentioned, but do have tightly-knit communities.
As a final note–here are a couple guides written by a prominent community member, that you may find useful. One is more geared towards writing character stories here on the forums, but is still helpful in regards to polishing writing in general.