Start with dorans shield. Yes this is simple but it works well. Next get kages lucky pick. This is good for gold generation and gives you 25 AP which is quite a bit for early game. Get the Sorcerers shoes next unless the enemy team has a lot of CC (then get mercury treads). Next get the giants belt and use it to build into Rylais crystal scepter. Make sure to get giants belt first so you have more hp and appear scarier to the enemies. The deathcap is a must have so get that next. the last item show is will of the ancients. This will help keep you alive and well because by now they should realize they need to focus you. The items after depend on how the game is going (this means selling your shield and pick). If you feel like you die too fast because your spell vamp is not enough, get hextech gunblade. You could also get the hourglass because of its active. If the enemy team has a lot of tanks then get deathfire grasp. Another good sugestion is getting Lichbane.

All of the runes are AP per level runes. This is useful during mid-game and greatly helps with the deathcap.

Make sure to grab all the masteries in the utility section not involving mana. The only one that should involve mana is the Perseverance (gotten for health regen). Also make sure to grab the magic penetration in offence.

Max out Thundering Suriken(Q) first. After get Electrical Surge(W) followed by Lightning rush(E). This is assuming you get your ult(R) whenever possible. Use your Q to last hit enemy minions early game. Also use it to harass enemy champions followed by your W, if you have your W passive built up you can use that as well. Do not use lightning rush offensively early game unless it is to catch up to get a kill. It should be used to run away for stun an enemy to save someone. Also note that it gives you magic resist and armor making it possible to block damage to save your life(if you play right you wont need it). For a team fight wait for someone else to start the team fight. If you start it you will not last very long at all. After the team fight has been started use E to rush into the enemy team and then through out your ult.