Select clothie. Cheap Shot, Mutilate, Eviscerate. Move on to next clothie.
Just Kidding! You might sometimes use Kidney Shot too!
Today I'll focus on Battlegrounds with a Mutilate/Prep spec, since it's what I've been doing lately. This is a guide for relative beginners, so don't think you're smart if you say in the comments "lol u didnt no abut [obscure advanced move x]! nubcake!"
First, I recommend speccing 41/5/25. It's aimed at pumping each Mutilate's damage as high as I can get it, and gaining energy to put out Mutilates faster, particularly in tandem with openers and finishers. Mutilate's damage was nerfed a bit in a recent patch, so other specs are starting to come into favor, such as Combat-based specs or Hemo Shadowstep specs, but I still found Mut/Prep to be very effective in BGs.
Most of the points here I'd consider non-negotiable. Yes, you want Two-Handed Weapon Specialization (Since Mutilate hits with both hands, this talent also increases the damage of your main attack). In the Subtlety tree, you can mix-and-match a bit with Elusiveness and Serrated Blades. In the Assassination tree, I put my optional points in Vile Poisons, Improved Kidney Shot, and Focused Attacks. You could just as easily prefer Ruthlessness, Blood Spatter, Quick Recovery (definitely take this over Imp. KS if you will be grouped with a healer!), or Deadened Nerves.
Glyph for Eviscerate, Vigor, and Prep (allowing for chain Dismantles).
The best thing about this spec is that it focuses all of your talent points into a small number of abilities that you will use in almost every fight, and doesn't waste much on peripheral moves. Not only do you spend a lot of points on buffing Mutilate and energy regen for more Mutilates, but you also stack bonuses that reduce the cost of the opening Cheap Shot (Overkill and Dirty Deeds) and buff your primary finishers (Eviscerate and Kidney Shot).
Also, don't skimp on the points in Master of Deception, Camoflauge, or Heightened Senses in the Subtlety tree. I know they don't seem glamorous, but being able to spot and outmaneuver other stealthers before they see you has untold value in PvP. Without these talents, prepare to be opened on by other rogues and druids ALL THE TIME. It won't be fun. You might also consider enchants or items that buff your stealth (not at the cost of too much damage, mind you) or the engineering goggles, which give you an edge in detection.
Two slow daggers are probably best for big Mutilates, but you can have a faster offhand without much loss due to the increased poison procs. Go for a 1.8 or 1.7 speed mainhand and a 1.8-1.5 speed offhand. The badge dagger works for offhand, and you can search daggers on wowhead to see where you can best pick up a mainhand. Since slow daggers are terrible for Mutilate raiding, it should be easy to pick up a nice slow dagger from raids.
Enchants are up to you (but you want an enchant). +50 AP is a strong option that is extremely cheap. If you want to splurge, Berserking and Mongoose are also strong, and +26 agility is another contender. As long as you have at least 5% +hit from gear, don't bother with Accuracy. I'm sorry to say I have no clue if Icebreaker is any good, I never risked the cash to try it.
Generally, put Wound Poison on both hands. Unless you spec deep into Improved Poisons, it averages out to more damage than Instant Poison (due to having a higher chance to apply) and includes the supremely useful healing reduction debuff. Unlike Instant, it also applies a poison debuff to the target that buffs your Mutilate damage. There's no need to use Crippling, as you have a talent, Deadly Brew, that causes Wound and Mind-Numbing poisons to also apply Crippling. If you expect to fight a lot of casters, you can go with Mind-Numbing on one weapon, but I generally don't bother in Battlegrounds. Casters are usually your easiest kills anyway, and Wound procs off your Mutilates will up your burst damage considerably.
First, stay stealthed whenever you don't need to be mounted. Get a lay of the land and choose your targets carefully: your stealth is your main defense! If you get caught out in the open, you are an easy HK, especially if your cooldowns are down. Remember that in the current environment, everyone, including you, dies in just a few seconds.
Your bread-and-butter opener is going to be Cheap Shot followed by Mutilate. This will give you 5 combo points. At this point, you have two options. If their health is low, you can Eviscerate (hit Cold Blood first for extra laughs! Generally, save Cold Blood for 5-point Eviscerates rather than Mutilates). If you need to keep them under control, use Kidney Shot just before Cheap Shot wears off. You want to have as much energy as possible saved up before you use the KS, because afterwards you have 6 seconds to unleash on your opponent before they wake up very, very cranky. Mutilate, Mutilate, Evis. If they aren't dead yet, you should probably worry.
I've had some good results with Ambushes on clothies, but in most situations the stun is more valuable, especially since our Ambush isn't fully talented.
Once you are out in the open or your stuns wear off, you have a number of tools at your disposal to stay alive. Keep your Vanish, Blind, Dismantle, Evasion, and Cloak of Shadows in mind at all times, and through practice learn when to use them. Blind is great to get someone off you while you finish off their friend. Evasion and dismantle are your main defenses against other melee. Vanish is your escape button. Try to learn how to use it pre-emptively, when you are about to be attacked. If you wait to long, you're probably going to get stunned and/or killed before you get another chance to hit the button. Cloak of Shadows probably has the most diverse uses. Get used to popping it just before a Vanish to clear off DoTs that might break you back out of stealth. Learn the right moment to hit it just before a Mage unloads fiery pain on your ass, but save it against warlocks until after they've applies their dots. It can also give you the edge in a head-to-head battle against a DK by removing his diseases. A more advanced move is to use it when first engaging a paladin, because it makes you immune to their stun, but you have to be able to predict ahead of time when they will use it (luckily, most use it at the beginning of the fight if it's off cooldown).
After your opener, you have a two options: fight of flight. You can either continue working on your current target and/or moving on to a nearby target without restealthing, or find a way to restealth. Keep in mind you are likely going to need to use some cooldowns to survive more than a few seconds out in the open against any opponent whom you do not have currently stunned. Most classes have the burst damage and defenses needed take you out before you can finish them if the stun wears off. Alternatively, you can run away from someone you've just killed and wait for yourself to drop out of combat so you can restealth without using Vanish, but this method is risky. Still, I tend to use it a lot. Interestingly, in a 1v1 situation you can drop combat and restealth while your opponent is Blinded, which is a very strong way to bandage yourself and restealth for another Cheap Shot or Ambush (provided the opponent doesn't trinket out of it). If you've just finished an opponent (or barely scratched them during the stunlock) and need to run away before they or their friends can retaliate, just pop Vanish and reset the situation so you can pick a better target or move on to a less dangerous location.
Also, get a 2-minute PvP trinket. You'll need it for 3 things: Fear, Stuns, and Blind. It will literally save your life against warlocks if you can break their fear and respond with a Cloak/Vanish/CS or Blind to get back in melee range. You give yourself a fighting chance against Paladins if you can trinket their stun FAST. Against a fellow rogue, save your trinket for the Kidney Shot (rather than the shorter Cheap Shot) and immediately Vanish/CS if you can. Against other rogues, the fight is all about who is most effective with their stuns, and Vanish/CS is key in this matchup. Against rogues I know to be skilled, I'll often open with CS/Rupture to prevent them from Vanishing and turning the fight around on me.
And thus ends my beginners guide to rogue PvP. Now get out there and kill people so bad they start threads on the general forums calling for rogues to be nerfed!