(Posting some tips for PvE players who want to PvP. Please read and comment, and more to follow later.)
I'm going to assume you don't want to drop PvE for PvP and go through years of GvG or other PvP formats. I'm also going to assume you want to Codex casually, and not quit your PvE guild for a Codex guild or PvP guild.
First, you must understand the fundamentals of the game. That is, what is damage, what is shutdown, and what is the point of healing. If you're mainly a PvE player and think you know what these are do yourself a favor and read through the articles on damage, shutdown and healing. PvP is very different than PvE and the other articles go into depth about these differences.
Secondly, because you PvE your friends are probably PvE. This has some obvious but often overlooked implications. The main implication is you cannot run the daily "meta" and win. If your friends are mostly PvE then your "skill" (if you want to call years of doing the same thing over and over skill) at PvP will be less than those who do PvP every single day. In game terms that means you need to make your team build easier to play. For example, if Ebon Dust Aura or Blinding Surge are in today's Codex, rather than relying on your monk's ability to draw or remove conditions which will obviously be inferior to a Champion rank monk, make your team immune to blind by default with Avatar of Melandru. Obviously relying on a necromancer to rip the enchants off the dervish or for a monk secondary to draw the blind is more ideal. But the fact is you and your team are PvEs and your reflexes, instincts and teamwork will not be nearly as good as PvP players. So what? Don't fight them at their strengths. Find their weaknesses, which due to the nature of the Codex Arena every daily meta will have, and exploit it.
My suggestion for PvE who want to try out Codex Arena is find one other patient PvE player to play with you. You along with him will form the core of your team. Generally healers are build independent. So what you will do is make an original build with your partner that works with one of the daily meta builds, pick up a monk and go. To be clear I'm also not talking about going on voice chat and spike chaining or anything so complicated -- just someone who is willing to put up with losing a few (or many) matches until you figure out a way to win.
To make an original build with a partner, you need a basic understanding of game mechanics, particularly what damage is and how damage is prevented and dealt. I'm not talking about running equations or spreadsheets or anything insane like that, but understanding basic game facts like when and how monks die, how damage is prevented and so on.
The point of this original build will not be merely to counter their strengths. Codex Arena can be a rock paper sissors game at times, for example with the earlier Melandru example. But if all you consider is how to counter this skill or that skill, you'll lose simply because you're being reactive. If the other team can force you to react to them instead of you forcing them to react to you, they have control. If they have control, your chances of winning go down. The point will be to render their strengths irrelevant, and play to your strengths. If both you and your partner are good at pressing 1-2-3-4 (and who isn't) use that. Make two uncomplicated builds that work extremely well like that. Take control.
An example of taking control is running a melee that can cripple their melee. Rather than ignoring the warrior or assassin at the start of the match like PvP guilds do, cripple it and keep it crippled. Instead of playing the way a PvP player would play and losing because they play it better, play your way. And when their melee gets frustrated because he's constantly crippled, knocked down, or blinded and decides to attack or spike you, you'll be ready with your blocking stances. He probably won't have a blocking stance and definitely won't have a heal because he'll be relying on his monk to heal him.
Now you're forcing them to react to you. You have control, and better chances of winning.
Another example of taking control in a different way is changing the "rules" of the game. For example, PvP players are very good at swapping targets as soon as a guardian comes up. If you swap targets when their monk is using guardian and they swap targets when your monk is using guardian, you're playing to their strengths. Instead rip that guardian off or make your attacks unblockable. Now you're making it a competition between your ability to see when they've finished casting guardian and pressing the number one and their ability to quickly assess and traverse the distance to another target then count the seconds until guardian wears off to switch back to the priority target. Even the best cyclist in the world can't beat a Ferrari.
That's only two examples of taking control. There's many, many other ways but the point is you do not want to fight experienced PvP players the way they play because they'll do it better. When in Rome, do not do as the Romans. Taking control doesn't just mean in-match tactics too but overall strategy. Run two healers if restoration ritualist and monk have decent healing skills. Run no dedicated healers if healing is incredibly awful as it was on October 25th 2009, and run four damage dealers. Rig the game.
I hope this article can take some of the frustration away from PvE players wanting to Codex. It may take ten or twenty tries with your partner before you start winning. You may have to leave the party and pick up a different third class and a different monk. You might have to scrap your build entirely and start from scratch with different classes (which can be a pain for PvE characters with no slots left but PvEs have three accounts and dozens of slots anyway.) The good thing about PvP matches is they last a few minutes. Twenty or even fifty matches isn't as long as a Mallyx, and after half an hour you should be able to invent a build with your partner that can win in Codex Arena, and meet some interesting people doing it.
Finally some tips to avoid hair pulling and smashing your computer:
1. Pick a good day. Good days are generally those with tons of damage prevention and healing. If you pick a day like October 25 2009 where the only healing was boon signet, it's a waste of your time.
2. Pick a weekend. Pick a weekend day where they'll be more PvP people willing to team with randoms. Also more people at your skill level to kill.
3. Pick a good time. Good times are during afternoon and late evening when all the kids are on after school. Bad times are late at night where the only people still playing would be elite guilds.
