Caitlyn is a ranged Attack Damage carry, a role that specializes in building massive amounts of damage and annihilating enemies from a distance.

Caitlyn's strengths compared to other ADCs are her considerably superior range; the average ADC has a range of 550 whereas Caitlyn's is 650, allowing you to hit your opponent without receiving damage in return. Caitlyn's abilities combined with this range allow you incredible poking and zoning potential and make her a very safe, strong pick that has very few losing matchups. Caitlyn is considered to be one of the best ADCs in the game at present.

On the downside, her mid game is somewhat weak and her aggressive pushing/zoning style of laning invites ganks from the enemy team. She's also not the best in a straight up duel to the death, so if you fall too far behind on Caitlyn you could find the game snowballing against you.

These marks are the most logical choice with this Lifesteal quint setup. You need the extra AD from these to help you last hit minions and give you increased damage on your harass. Alternatively you can use armor penetration runes here but if you choose to use them you'll have a difficult time with last hitting.

Armour runes are self explanitory, the metagame will have you always laning against another AD carry (or possibly AD bruisers) which makes armour runes required unless you want to take huge amounts of damage and die a lot. Never play without these as soon as you're summoner level 20.

MR runes are simply the best remaining choice. There's really not much that's useful for an AD carry in glyphs and you're extremely squishy to AP carries so a little extra resistance there won't hurt. Take flat MR if you're against a bot lane with strong magical damage - champions such as Sona, Lulu, Corki, Zyra or Tristana are good examples of this. If not, you can take MR per level instead. If you're not sure you should stick to flat MR - most bot lanes deal enough magic damage to make using per level risky.

These quints are absolutely amazing and have been underestimated for a long while. With these combined with Doran's Blade your sustain is very high and in many situations you can actually skip buying lifesteal until much later in the game. Being able to sustain from the start of the game at zero gold cost is awesome and Lifesteal also scales into lategame better than any other rune. The alternative is using attack damage, which will give you a little more damage on your early harass.

The reason why I prefer to start with Q is because it gives you the potential to hit much harder at level 1. Landing a perfect Q can win your lane immediately and it's much better if there's any kind of confrontation. Starting with W is somewhat popular but I feel it very rarely pays off and leaves you extremely vulnerable and unable to push early.

Since your W and E skills are mostly for utility you'll want to prioritize points in Q first for massive damage on your poke. I prefer to level E next, because it gives you more escapability and it gives you more damage + a faster cooldown on your burst combo. Skilling W can make your traps more annoying and let you spam them more effectively but I think it's generally not as good.

As always level R whenever you're given the option.