Don't forget to kill minions with your 1st spell, you get extra gold. And when you attack turret or ennemies with team mates your 3rd spell gives a speed buff to everyone. The best way to play gangplank is make lots of gold early and get heavy damage items aswell as Attack Speed. When you want to buy the last Phantom Dancer and have no room in your inventory, you can sell your boots, at this point they are not very useful. So at the end try to have 3 Phantom Dancers, 1 Bloodthirster previously stacked, preferably, during the game and Infinity's Edge. Nice damage output.. Fast, efficient and you steal their life while you're killing them =)
Enjoy

P.S : not too sure if the skill sequence is correct, but it gives you a rough idea of what to do