Hey.
So this is a pretty straight forward Caitlyn guide. I won't go too in-depth because really all you should need to learn from this guide is how to pick masteries, runes, and skill sequencing. Everything else you should already know (last hitting/farming properly, position in team fights, all the necessary ranged AD requirements). If not, I might update this guide later on.
I made this guide because of how many people I see picking Caitlyn and then playing her wrong, not even close to her full potential. So anyways, here it goes... have fun raping face as Caitlyn, gimme upvotes and comments but be sure to read through what I've written (as little as it may be).

random: incase you didn't already know, caitlyn should be bottom lane with a support sustaining her. Don't go solo top against a garen or mid against a morgana :/ That's not too smart

When it comes to runes, I like to change it up a bit once in a while.
When I see the enemy team picking decent champions, I'll go for my usual +25 Armor Pen runes. But if I see them picking a naturally squishy team and a bad bot lane combo, I like to have fun and go for my full Attack Damage reds + quints rune page, get a dorans blade, and start off with +28 attack damage to really stomp the early farming phase and out CS their ranged AD carry by tons!

As for my blues and yellows, they stay unchanged. I hear people talking about and suggesting mana regen per level runes and cooldown reduction, but why?
Mana regen per level is pretty useless, it got nerfed a few months ago and hardly gives you enough mana regen to make that big of a difference until like level 10. But even then, you should still be farming at level 10! Not wasting mana spamming spells and pushing waves and missing last hits. All the mana you need is traps to have bush control, traps to help see incoming ganks, and nets to escape. Occasionally you want to land a Q directly onto an enemy champion for that ridiculous first hit damage, but if they're behind minions, it's not worth wasting the mana. Farm up, out CS your enemy ranged AD carry, and you've already won. Stop wasting mana!
Flat cooldown reduction. Again, useless in my opinion. 5.89% CDR at level 1, for what? A few seconds off your ulti? Your ulti isn't that much of a team-fight game changer anyways, so it's not like your team needs it like they need Ashe's ultimate. It's better if you're laning and somebody escapes, bam, ulti, and that's it. CDR runes, completely useless.

As for my runes, they go great with survivability. I picked flat armor because you need the armor early game. Your base armor stat is very low and most of the damage you take in the early laning phase is physical damage, so why not? Along with your wriggles, that's around 119 armor at level 18. With only 1 item and your runes, that's almost 55% reduced physical damage. How can you refuse this?
Magic resist per level is also great. +24 Magic resist at level 18, paired with a Banshee's Veil, you're at 104.3 magic resist. Once again, that's more than 50% reduced magic damage with only 1 defensive item.

I like to run 15/0/15 or, most of the time, 14/0/16.
Why?
A full offensive mastery tree is completely useless. 4% increased physical and magic damage is nothing. 10% increased physical damage is also trivial. These things can all be obtained via items. Almost everything you can get from the offensive mastery tree, you can also get in-game just by opening up the shop and buying a cheap item. The same can't be said about the Utility Tree, though. The utility tree has great boosts that can only be obtained via masteries, not items like the Offensive tree. Decrease flash cooldown, increase move speed, increase experience gain. All of these things are great to have. Heck, you might even spec 9/0/21 once in a while to make sure your Flash is up almost every team fight! Positioning is KEY when you're playing ranged AD carry, and there's nothing worse than getting caught out of position in a team fight while your flash is on cooldown and going down without a fight. Your team now has to fight 4v5 while their main source of damage is missing.

My build on Caitlyn is pretty straight forward and hardly changes.
First item Dorans blade.
Depending on how well you're farming and your support is sustaining you, you should be able to stay in lane without missing much creep score or letting your turret go down until you have enough to buy another dorans blade, boots of speed, and almost a full wriggles lantern. The wriggles lantern isn't a requirement, though. It's great for a well coordinated team who are keeping timers on Dragon and gets them done as soon as they come up, as the proc on you and your jungler who likely has one too will quickly finish up the dragon in seconds (not to mention baron!). Although if you find that you don't need a Wriggles, I usually opt for berz greaves, 3 dorans blade, BF sword. Those items, and you basically become unstoppable if you've outfarmed your enemy ranged AD carry.

Obviously turn the BF sword into an Infinity Edge. Then build a zeal, turn that into a phantom dancer. If they're really tanky and you're having trouble doing any actual damage, get that Last Whisper. If you find you're doing extremely well in team fights, shredding everybody apart because of great positioning, but you get caught once in a while by their AP carry, get a banshees veil before your next damage item, but that almost never happens. I usually almost always get the last whisper before the BV, but sometimes you never know, those suppressions/stuns/snares can really change a game completely!

Not much to say here, except one thing.
Never get your Q first! I absolutely HATE it when people get their Q first. There is absolutely no reason to get your Q first. Get your trap first! Traps are useful in not only protecting your jungle, but taking control of the bush in bottom lane even at level 1! Your Q does around 80-90 damage at level 1, less some because of armor, and you're left with, assuming how tanky the person is, around 65-75 damage. That's not much more than an auto attack. Wouldn't you rather have your enemy run onto a trap in the bush and get stunned for 2 seconds so you can land 2 more auto attacks for 55+ each than have your Q at level 1 that isn't even guaranteed to hit? Yes, it's hard to miss with it, but you get my point. There is really never a reason you should be getting your Q first. Help protect your jungler by trapping jungle invasion routes, and gain brush control of bottom lane for the first wave with only 1 trap!

That aside, max out your Q. Then max out your E (net) for decreased cooldown and increased slow duration.

Flash is a must. E over a wall, flash over another one. Extremely hard to catch (unless you're running from a Kassadin, then... good luck :p)
I normally get exhaust because it's great for avoiding ganks from a strong jungler. It happens to all of us, we're slightly overextended and eager to get some damage on their turret and bang, Gangplank or Nocturne comes creeping out from behind. Flash behind him, E away, and make a run for it. If he's still behind you and might grab a kill, drop that exhaust and laugh at how slow he is and how little damage he does :p. Also, it's amazing for fighting 1v1 when both of the supports bottom lane are missing! RAPE THAT AD CARRY!
It's also amazing in team fights. If you find there's an assassin on you like Akali or something, just cutting away at you, drop an exhaust and yell for your teammates to come and kill that thing before he finishes you up.

I love to play Caitlyn in ranked games. I've carried a few with some amazing scores, but I don't play ranked that often.
When you play ranked, be sure to communicate with your team during champion select to get a great support to bottom lane with you. It's all about coordination and communication to win ranked games. Communicate with your team members, let your lanes know when somebody bottom lane is missing (like an annoying taric with an annoying stun who's gonna go mash on your mid lane), keep timers on Dragon and push out your lane to the turret all the way and go help your jungler finish it up really quick whenever he's doing dragon. If you have a wriggles, ward the tri bush or the dragon/blue buff entrance to avoid ganks. If not, remind your support that your wards have expired. Communicate, and coordinate... and win games!

Great laning partners are key. You need to have a good chemistry with your laning partner to dominate the laning phase in terms of both farming and denying.
I find my favorite lane partner is Taric. He's great at sustaining my health, and has an AMAZING harass. We communicate who we're gonna harass, he initiates and stuns just as I lay my trap down under the guy. Stun goes off, trap goes off shortly after, that's nearly 3-4 seconds of stun which results in loads of auto attacks, bursting by Taric's shield, and me landing a nice little Q just as soon as they're running off :) they either die or are forced B, denying them of experience and creep score.

So try and find somebody who you can lane well with. If you want to focus purely on farming, you still need somebody who can keep the aggro off of you, you can't just have a healer keep you healed all laning phase... you might still get denied!
I find my favorites are:
Taric
Soraka (great harass and poke with her silence and starcall, and can keep me full on both health AND mana)
Janna (can keep me safe from everything. Ganks, dives, all that good stuff. has a shield, a slow for kiting or escaping, and an AoE knockup.)
Lux (she can't keep me sustained but her shield is pretty useful. as for harass, it's pretty ridiculous. lux alone can do enough damage to force their ranged AD carry out of the lane, but toss in some auto attacks and Q's from caitlyn while they're snared, and boom. goodbye! not even a soraka can outheal those kinda pokes, so again, she's forced to either hug turret and lose experience while I last hit, or go back and again... lose experience :p)

That's all I can think of.
Turned out to be a lot more than i thought it would be :s anyways. Have fun. Gimme some votes and leave a few comments with your thoughts.
Have fun destroying as Caitlyn!