Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is an AP focused/pseudo hybrid build.
Get as many minion kills as possible. Use Crescent Slash combined with the Tome that you bought first to get multiple mob xp early in the game. You can harass the enemy a little, but your focus is on leveling near your tower. You will also buy Kage's Lucky Pick to get more gold throughout the game, and in the end will sell it once you reach level 12.
You get Shadow Dance... the gank spell. Now you can try and kill the enemy, but still be careful. Continue to level and use crescent slash to xp quickly. Also, don't forget to use Twilight Shroud to get out of sticky situations or to slow the enemy down when they are near your tower. And this is when you will get the Berzerker Greaves. The faster hit speed will help a lot.
This is where you can start supporting your team and plan your attacks. Level up and get as much gold as possible. Jungle if you have to. Your goal is to get Rylai's Crystal Scepter and Lich Bane ASAP. Then you will start dishing out tons of damage.
Same as before but now your last few items are dependent on the state of the game. If you're dying a lot, get Sunfire Armor... if not, then focus on getting Zhonya's Ring. If you want to help your team more, then go for Stark's Fervor.
Enjoy and kick arse!