My strong recommendation is to look up a good hunter PvP guide for your preferred spec, then. There's two hunter PvP guides stickied in the WoW EU Hunter forums, one of which is labeled for beginners!
http://eu.battle.net/wow/en/forum/874791/
That said, some random stuff I've learned:
The whole idea in hunter PvP is to avoid taking damage while dishing out heavy damage yourself. Spec for Entrapment and defensive abilities like Crouching Tiger, Hidden Chimera (for BM), Survival Tactics, & Hunter vs Wild (basically anything with reduced defensive cooldowns, more snares/roots, stamina).
Hide in bushes, on roofs, whatever, but always watch your pet's pathing--if you're BM and try sniping from a roof, your pet will be stuck up there with you and you're crippled. But the point is, try & stay unobtrusive. If you try hunter PvP in the thick of things (like I tend to do, lol >.>), chances are you'll get focused and destroyed pretty quickly. (Note: if your pet DOES get stuck somewhere, and you're in combat, click Follow manually and the pet should teleport to you, allowing you to send it after the enemy.)
Obviously you want to get some
resilience gear asap, but don't worry, hunters can be pretty strong even without gear if you play them well--your entire goal is to dish out damage while not getting hit, so if you are doing it right, it isn't as painful as trying to play, say, a warrior with no gear. That said, better gear certainly does make a difference down the road in scoring kills! Full raid-geared hunters can kill someone in a silencing shot, but they go down mighty quick when focused, too.
Macro /focus and bind it, and macro both offensive and defensive abilities to be cast @focus with a modifier, if possible. This way you can one-keypress focus a friendly healer to provide Master's Call/Spirit Mend/Roar of Sacrifice support, or focus Scatter Shot an enemy healer while burning down a dps.
#showtooltip Spirit Mend
/cast [modifier:shift, @focus] Spirit Mend; [nomodifier, @target] Spirit Mend
^ is an example.
Bind as much as you can--I don't know if you're a clicker or a keybinder, but macroing and binding everything is a must, really. Hunters have more abilities than any other class I've played, so binding everything can be tough, but it really makes a difference in performance.
Protect healers. If you have one that's tossing you a bubble, throw a trap under him when a rogue opens. One of your greatest strengths as a hunter is the ability to peel, so use it to help the healers out, and they will reward you by keeping you alive.
Feign Death to drop an enemy caster's target (as an interrupt!), or to make a warrior who's about to charge lose his target & spend precious seconds finding you again. Drop Snake Trap with Entrapment on a melee, then feign and run off; they will have to target you past the snakes, which is irritating at best.
Tranquilizing Shot works WITHOUT breaking cc. If you find yourself 1v1 versus a healer, paladin or frost mage, CC them if you can and then purge off everything before you open (and open starting with a focus regen shot). This means that when they pop Avenging Wrath etc, you can purge that straight off without having to chomp through trash buffs first.
Disengage can be used to avoid fall damage if used at the bottom of a fall while in combat. It can be used to pursue your enemy (by spinning, disengaging & spinning back to face your target) and can also be used to get back onto a ledge if you get knocked off of it. You can also deliberately jump off of a ledge if you're being chased closely by a melee, and then disengage back as soon as you're in the air--and sometimes the melee will have jumped after you, and you'll have the high ground with them falling or trapped below. I suggest lots of practice using Disengage to simply move about.
If you don't have DoTs on you, you can trap +
bandage up. Have First Aid maxed, if possible, for hunter PvP. You won't often get a chance to do this in the heat of a bg, but when you can, it can be a lifesaver.
If you're 1v1 versus a healer, feel free to use your scattershot as an interrupt even if they've got DoTs on them; you can pop it right as a heal would finish and then just continue nuking.
Anti-Stealthy Tips:
Watch enemy AREA buffs! By this I mean, if a priest is running toward you, look for Leader of the Pack (the orange/black bullhorns icon) or Master of Subtlety (gold and black ninjastar-ish thing) on them. This will immediately clue you into whether there's a rogue or feral druid stealthing up to you. Keep flares down as much as possible, and if you're guarding a base while mounted, be ready to click off your mount so that your pet can get some hits in if you get stunned by a stealthy while mounted. I'd stay camod slightly away from the flag or base, with traps laid on the node. When a rogue CloS-es, stun or scatter it and immediately reapply Hunter's Mark the second CloS is off--and Serpent Sting it, and Flare in the direction it's facing, just in front of it.
Lastly, depending on what spec you choose to PvP as, here's
some pet options; bear in mind that you want a Cunning pet for the most part, or even Tenacity, unless you're BM going for heavy, sustained damage. Even so, a Cunning pet is good for a low-resilience BM hunter due to the Roar of Sacrifice ability.
MM/SV:
* Spider: Ranged root.
* Monkey: extra Scatter Shot.
* Crab: melee root, added bonus of Intervene.
* Bird of Prey: Disarm.
* Gorilla: Interrupt.
* Bat: Stun.
BM:
* Spirit Beast: Heal/damage bonus (my favorite by far)
* Devilsaur: Mortal Strike/damage bonus (close second)
* Silithid: Ranged root, +stamina bonus
* Chimaera: Near-constant ranged snare
Which you choose will, like your choice of spec, depend on your playstyle and preferences! Speaking of which, I had a blog post ages back about BM PvP, so if you choose to go BM, check it out--it goes into a bit more depth.
Clicky!
I'm sure there's an absolute ton of stuff I'm missing, but these are the basics that come to mind. I can't really speak for MM/Surv PvP, so I won't try

If you choose to go BM and have any questions shout and I'll try to help, otherwise I'm sure there's MM/SV PvPers on the forums somewhere
