Hello 
This is a long post.
I can only speak for Argent Dawn which is by far the most populated server RP wise, there is a guide posted there with basic RP information (see below). On that realm Alliance is the biggest faction, not to say Horde don’t have RP as well because they do, they are just smaller. Human is always a safe bet but I’d say you can find RP as any race, for class, maybe don’t start with one of the “harder” ones like DK or DH that are pretty niche compared to others, this could include Void elves as well.
Argent Archives is a website with information about events and guilds for Argent Dawn, and you log in by creating a page for your character.
A general rule is that “/say” is IC (In character), the Trade chat is more or less the same wherever you go, and if you take Stormwind as an example there are areas (e.g. Trade district) that most ppl are OOC (out of character) but Cathedral Square is an area where more people are IC, so there’s a mix and you will see the difference.
A popular addon is TRP3 (Total Roleplay 3), in which you can add information about your character like birthplace, a background, height, eyecolor, etc. and something called “glances” which are a perfect way to let people know these first things they might notice when looking at your character. Look at others to get some tips on what to put in there
If you join AD (Argent Dawn), keep away from Goldshire.
Check out wowpedia to get a basic grasp of the lore, (it’s more updated than wowwiki)
Look around for what different kinds of guilds there are and what type you should want to join (if any), either on the forum or AA (Argent Archives)
Throw yourself out there, say hi, most people are nice and helpful 