This is a bit Different from most GangPlank guides. this is a build i like over just buying a bunch of Crit items. that at level 18 stats are bugged that are a littler higher than what is said. In Game you are at a little over 3,000 health and 400 AD.

The tunes i picked are simple to explain.
http://edge4.mobafire.com/images/rune/greater-mark-of-furor.png Obvious More Damage
http://edge4.mobafire.com/images/rune/greater-quintessence-of-furor.png Same Thing
http://edge4.mobafire.com/images/rune/greater-seal-of-vitality.png Gives you the Health Needed so you wont have to buy any health items
http://edge4.mobafire.com/images/rune/greater-glyph-of-focus.png Little bit of Helpful CD Reduction

Basic Ad Carry Masteries

Dorans Shield to start you can take ring but i like the Health Regen and Armor.
First time i go back i like to make sure i have enough gold for rank 1 boots and Sheen. Sheen is very important gives you Much needed Mana and Doubles your Q Damage. Next time i go back i like to make sure finish my boots i like boot of Mobility because i like to get around the map and Gank Once my Turret is down.i will keep the rest of the items on my list except for Tiamat. The reason i grab this item is because 1. It helps farm minions waves quick and 2 because the effect is very helpful in teaam fights as Gang Plank Splashing your Q dmg is amazing as is when you run up and Auto Attack this works very well especially on Teams with Multiple Melee Champs or ranged champs that are standing together. have had this work very well With Viegar Amumu and Galio if they can get people stunned/suppressed/ taunted together they are as good as dead with 1 q and a couple auto attacks. Before You say "WTF TIAMAT?!?!" give it a try it works very well

This Build is Basically a Heavy Dps Build. is use heavy as (not so easy to kill) you have basically a little bit of everything. lets go over it
You have roughly 50% crit
a good chance to Proc your slow
Very decent life steal
High AD
and High Crit DMG

Over all i suggest you at Least try this Build and Let me Know how it Works for you.
I left out Basic Game Play info as if you are reading this i am assuming you already know how to play GangPlank and are just looking for a way to build him better.