Oh and, Get a good spec. Go to arena junkies and look at the top DKs, check out their Unholy specs, (dont play as frost, unholy is easier to get used to and less gear dependent.) Do what they do.
Use pestilence to get death runes, but use your best judgement. Obviously if you are attacking a DPS while the healer is cycloned, and you want a kill, you want to do damage to them so you would blood strike instead.
There is no rotation in PvP, and there isnt really one in PvE either unless you are Blood. DKs are a very dynamic DPS class and the best attack to use isnt determined by a single factor, but multiple. Just scourge strike over and over, apply diseases when you need to, and spread them to all targets for more pressure on their healer.
You aren't playing a ret paladin, so you dont just judgement crusader strike divine storm over and over in some specific order, you manage your rune cooldowns and diseases to set up burst damage and be aware of what is going on around you to know when to use each of your many cooldowns.
Try and queue during times when most other geared players arent. A good time to do this is on monday night, before tuesday, when all the trade channel arena pugs do arenas for points with wacked out comps and bad players.
Duel often when your partner isnt on and you are waiting around or not doing anything. This is the best way to learn how other classes work in PvP and how best to react, duels may not be indicative of how a class plays exactly in arenas, but you will get an idea of how their abilities work and improve your reaction time and understanding of how to deal with these abilities. Duel good players. Lose a lot. Think about what you did wrong and how that player might have played differently when your resto druid and his partner is thrown into a 2v2 setting. Ask him why you lost and if he has any insight on what could have been done better. If they call you a scrub and talk shit, move on to the next guy that is willing to offer insight as to why he kicked your ass.
By dueling a lot, I have learned how to deathgrip a rogue right when he vanishes, breaking him out of stealth instantly. Little tricks like this can win arena matches or sway them heavily in your favor. So duel a lot, and learn from them. Don't kick the same player's ass over and over, fight players better geared and better skilled than you. Fight frost mages. Learn when it is appropriate to strangulate to save your ass (when the mage can just ice block it) and when it is appropriate to use it offensively to keep the mage from escaping your onslaught of scourge strikes. These are just examples.
Good luck and I wish you the best. Don't pay attention to the nerds on these forums, just keep practicing and gearing up, and when you can't play, read about it and make sure you are making the right gear choices and the right decisions in arena. Practice and communication, and you will get better.