You first start buying meki pendant, believe me, you already have enought AP for kill with just a Drain with ignite in lvl 1, this guide could be used to line with another character but I prefer take the mid to get lvl quick and gank both sides.
some tips:
-Hide over the keeper below middle turret, when ur opponent attack ur turret, use the Ultimate, flash, fear, drain nd ignite.*Follow this order* and the opponent turns a sh1t in front of you.
-You can jump with the ultimate even the enemy is around his turret, The enemy will think you are scared to jump but do it use fear, drain nd ignite, when you had killed the enemy, leave the turret using flash ;) very efective, FIDDLE ROCKS

Rabadon's Deathcap is all about boosting your AP, and while I miss having the Zhonya's Ring active available to me all the time, this item is simply more consistently effective in most situations. The vast majority of the time you are going to want to be casting Dark Wind, Terrify, and Drain when you pop into an opposing team - not using an active ability that immobilizes you while you are invincible. So now you have to carefully weigh whether you need Rabadon's.

Since this item was introduced in patch 1.0.0.111 and then buffed in patch 1.0.0.112, I have used it in almost every single game I have played. Mana regeneration, 20% cooldown reduction, and 75 AP make this a great Fiddle item. I don’t usually build it fully right away, as just a Fiendish Codex is extremely helpful in all of these areas for just over half the price.