Welcome to my Caitlyn build. This is my first build so criticism is She is a very underused champ that is hard to master, but once you do you can carry a team to victory, but only if you have a good tank. She can hit like a truck and has various abilities that work very nicely with keeping her in position. I main Caitlyn, and have since she was released, since then she has never gotten nerfed due to her severe under usage so Riot has not noticed some imbalances she has.

On Caity, I use flat Armor Pen Reds, and Mana Regeneration per 18 Seals/Glyphs. The reds for early game domination and something to eat up those pesky tanks. If you haven't noticed, Caitlyn is very mana dependent, hence the reduction of skill costs she got near the 3rd quarter in Season 1. Using regen per 18, her mana pool is almost always full.

On Caitlyn, go 21/0/9 to give her the maximum damage output and some nice passives. In the utility tree, feel free to put a point into Utility Mastery for the buff duration considering that Cait is lethal with red, but through multiple games, I've found that I prefer a point in Greed because the build is expensive.

Most Caitlyn builds suggest a Dorians Blade for the damage, the health and the lifesteal, what more could you ask for? Well going with a Dorians Blade may sound like a good base item for Caitlyn, and it is, but the more beneficial choice is going for a Regrowth Pendant and a Health Potion so that you have more lane durability, something that most opponents lack. At about level 6, or at 715 gold, B out and buy Philosopher's Stone and Boots of Speed and teleport back to lane so you don't miss out on experience. From here on in, B out when convenient. If you are mid lane, gank when possible and ensure the enemy will not escape using Caitlyn's Ace in the Hole. After completing the core build, ( x2 B.F. Sword and a Zeal and upgrading one of the BF Swords into a Bloodthirster) you can go 2 directions from here, you can continue with the build above, or if the enemy team is stacking armor, build a Last Whisper instead of the third Bloodthirster. If you need survivability, swap out a Bloodthrister for a Frozen Mallet and if the enemy team has tons of health, once again, swap out a BloodThirster OR in unlikely cases where more than just the tank has a Thornmail, swap your Infinity Edge for a Madred's Bloodrazer so that you have the extra life steal.

Easy skill sequence. Quick to memory. Your first priority in maxing is of course, your ulti, Ace in the Hole, then your Q, Piltover Peacemaker, then your W, Yordle Snap Trap, and last but not least, E 90 Caliber Net. Early game, your skill sequence should look something like:
Piltover Peacemaker
Yordle Snap Trap
Piltover Peacemaker
90 Caliber Net
Ace in the Hole
Piltover Peacemaker

In order from most recommended to least:
Most Preferred:
Ghost For chasing, escaping, and getting that last bit of needed range.
Teleport For lane dominance, back dooring, and assisting turrets/teammates in need.
Clairvoyance Another great Cait spell. Useful for getting sight for her ulti and getting info on the enemy team, don't forget to use this spell on the enemy team's jungler!

Possible Spells:
Exhaust For slowing, and lowering DPS, useful spell but Tele or CV is more useful.
Clarity Mana for yourself and the team. Its unneeded because of your runes.
Flash Good spell for Caitlyn, although it becomes useless once you can use your net wisely.
Ignite Would be a good spell. BUT the ignite range is closer than her auto attack range, putting her in danger to use it. If you want to use it because of the 50% healing reduction, buy an Executioner's Calling instead.

And as for these, no. Just no.
Cleanse Unneeded if you keep your positioning
Fortify Tanks use this not carries
Heal This is what your lifesteal and regen is for
Promote (How can you use this without hacking?)
Rally (Useful if you rush the mid turret at 1:00)
Revive Only viable on Karthus
Smite (Use only if you are Jungle Caitlyn)

Here's a simple list:

1. LAST HIT DEM MINIONS!
2. GANK DEM LANES IF YOU ARE MID!
3. KNOW HOW MUCH DAMAGE YOU DO SO YOU CAN SECURE KILLS DO NOT FORGET THIS!
4. HARASS THIS IS WHY SHE HAS RANGE!
5. IF THEY SAY SHES OP YELL "THEN WHY YOU FEEDIN' ME!?!?"
6. FOCUS THEIR CARRIES AND DON'T INITIATE IN TEAMFIGHTS!
6. FOCUS DEM CARRIES AND DON'T INITIATE TEAMFIGHTS I SAY THIS TWICE, ITS IMPORTANT!
7. IT IS OKAY TO USE YOUR ULTI FOR LANING HARASS
8. WATCH OUT FOR CHAMPIONS WITH A PHYSICAL DAMAGE SHIELD i.e. BANSHEES, PANTHEON, SION, SIVIR, HEXDRINKER, AND ZHONYAS, They can block your ult.
9. FOLLOW THE SUMMONERS CODE!
10. DON'T TALK IN ALL CAPS UNLESS IT IS TO SAY "IM ON MY BROTHERS ACCOUNT HI"
11. KILL DEM TURRETS!
12. IF YOU PLAY IN RANKED, STAY NEAR TANKS!
13. Always watch your positioning.

Caitlyn is a tough champion to master, even if you do bad on you first few games, as long as you enjoy playing her, keep playing her, and get better at her. Feel free to change up the build, this is just what I have found works best with my playing style of her. One more thing, since this is my first moba build, please criticize so to make this build better.