While I might not be a veteran of role playing in WoW as I have done very little of it I do have quite a bit of experience in role playing. My experiences, while different, still touch on the basics of role playing.
When you are new to role playing it is more about finding your footing and I am going to present a few tips that will help you do just that in any multi player venue you might want to role play.
1) The Basic Knowledge
If you are serious about role playing in WoW the first thing you should do is get a grasp for the basic knowledge of the class and race you wish to play. Class you can wing it with because you learn your skills as you go so it is rather simple. Don't have a skill yet? Play like you have not learned it, heck you can even play like you have never heard of it and how cool it sounds and you can't wait until you are skilled enough to learn it. Not really hard there right?
Race is a little different. If you are rolling a troll the absolute least you should know are the troll starting areas and maybe even a few names of the lore characters there. For a nice touch you can even try to remember the names of your class and profession trainers from the starting area. You should be able to spell these things too. I know it is only role play but nothing shows a lack of preparation if you can not even spell your home towns name.
You do not need to know the history of the troll race. You do not need to know the history of their relationships with the other races. Actually, you do not need to know a god damned thing outside of the starting areas. Unless you're role playing as a history scholar or a politician or someone that should know about these things it is not imperative you know them.
Better role players or people that really wish to know a lot will of course know everything there is to know about the troll race and its history. That is fine for them but as a beginning player it will be easier to keep it simple and just make your character a normal person and not some world scholar.
One note I must point out however. Remember that your characters are NOT you. If you are level 30 your character might have heard of northrend but it will surely not have any first hand knowledge of it.
Role playing without any interaction can have its place but the key to real personal story development is to interact with others. There are a few things a new role player needs to remember when interacting with others in any form of role play and the one biggest blunder by new players usually comes directly from here.
You do not know people that first time you meet them. This is something I have seen hard core role players mess up on. It is something that bugs me to no end when I role play. You can't just walk up to someone and say "Hello Seth" if you have never met them or there is no reason you would have known their name is Seth. Just because you can read their name doesn't mean you know it.
The same goes for titles. The same goes for guilds. Just because these things are listed over a persons head in the game it is not a license to assume you know them. If someone does not tell you information you do not know it.
With that said, address people as sir or ma'am or what have you until you are told their name. Do not mention their name, title or anything else while speaking to them until you have been told it or given a reasonable belief that you overheard it elsewhere and can say where you overheard it.
Keep all your interactions within the knowledge range of your character. Just like you might know the character is 85 and of course has been to twilight highlands you can not ask if they know "how the fishing in twilight highlands is". You can ask if they have ever been there and then ask about the fishing. Keep your interactions fresh and do not include any information you can get from looking at their character out of it.
3) Don't Be The Good Guy
No, I am serious. If you are new at role playing it is a lot easier to play the bad guy than it is the good guy. Anyone that has ever role played seriously will tell you that working stories as a bad guy is so much easier. If you are new and want to break into role playing, being a bad guy will make your life so much easier.
I am not saying that you have to be such an evil jerk that was so bad he was kicked out from hell but being a bad guy does give you the ability to cover up for a lack of knowledge, or the fact you have knowledge that you should not have had among other things.
A conversation about a lore character you should know pops up, playing a bad guy makes it easier to end the conversation. Just say you hate them and would rather not talk about them. If asked why, being the bad guy that you are you can always snap back, it is none of your business.
Caught accidentally talking about something you are not supposed to know? You are the bad guy, you have lots of underground sources that feed you information for a small sum.
Not knowing something that you should know? Play it coy but play it coy as if you are being hush hush about it for a reason. As a bad guy people will just think you are up to something by hiding information they know that you know.
If you play it as if you are always working some angle for your own personal gain it almost gives you a license to speak about some things only when you want to.
4) Getting Started in Group Settings
When in a group setting or meeting people for the first time as a new role player it can be really hard to find your place. First things first, do not interject yourself into someones role play by commenting immediately. If you feel the need to join a conversation start with an "excuse me but I overheard you talking about..." and you will either be invited to the conversation or asked to mind your own business.
The first few lines when breaking into a group role play are always the hardest but remember that not all conversation needs to be said. What? How can a conversation not be said?
There are emotes for a reason. Emote what you are doing. Looking up and noticing the color of the leaves on the trees have changed might bring someone to ask what you are looking at. Noticing the dagger on the rogue from a distance might bring the rogue to ask you what you are staring at. Things like that.
You can have an entire emote role play all by yourself while looking around and it is often the best way to get invited into a conversation or strike up a completely new one. If you are very descriptive in what you are seeing and how you are seeing it in emotes it will also let the people around you know that you are good at making things up on the fly which is a great trait for a good role player and could make them more interested in talking to you and inviting you to their role play.
5) Remember it is a Game
This should go unsaid but it should be mentioned that this is a game. Sure you might have killed deathwing but so have millions of other people. Deathwing is one person and there is no way in hell that millions of people have killed him.
This means don't go around bragging that you have killed deathwing. Each server seems to have their own way of handling this situation but what seems to be common is that the guild (and people) that got server first are considered the people that killed deathwing for role play purposes.
Mind you, I could be wrong here as I only have characters on two RP servers so I am not versed on all of them in any way shape or form, I am a WoW RP rookie. But that does not change the fact that usually it is a safe bet that you did not kill deathwing because if you did do so on a role play server you would probably have more knowledge of how they handle it on that server already.
You can say you ventured to DS. You can say you have fought big baddies there. You can say things like that but the big name, that one huge kill that ends it, that one is always reserved for the appropriate people to role play they did it. Just because it is a game where everyone can kill the big baddies doesn't mean everyone can role play as if they are the one person (or group) that did it.
Tips for Beginners
1) Take Notes:
Seriously. I have a book with information I need. Names of places and lore figures. Names of people I have met and what information about them I know. Notes about things I know that my character doesn't know but have a bearing on how I should act. No matter how good your memory is you are bound to forget something. Notes help a lot.
2) No Internet Slang:
I might be alone on this one but there is nothing I hate more than internet speak when role playing. If you can not picture your character saying BRB don't type it. Type be right back. It is to or too, not 2. Unless you mean two never use a 2. For does not equal 4 even if they sound the same. Honestly, if you can't be bothered to type correctly, don't become a role player.
3) Be Quiet:
When new it is one of the best ways to learn. Learn by listening. It might sound stupid but put yourself AFK and listen to how other people talk, it can be a powerful learning tool and being AFK will keep people from dragging you into the conversation.
Well, that's it for now. I am sure people that are more versed in WoW role play can give you more in depth advise but as a role player for many years I have found that these rules seem to follow in all versions of role play and it might be able to help you when you are new.