1- we need to gain exp for Ultimate, so we put Q skill.
2- we need to take damage to the enemy, so we put 2 skill.
3- for survive if we have to run away
4&5- upgrading W to shudder the enemies
6- Help with enemies going away.
REST-we upgrade to 5 the W, then the Q and finally E (always that we can upgrade R)

First we buy Dorian's ring, that give us more ability power, life and regeneretion (important)
Second we buy boots, essencials for run away or follow an enemy.
Third we buy Hextech Gunblade, that give us AD (to destroy fast minions and torrets) and AP (kill chanpions)
Fourth we need life, because the enemies kill us too fast, so we buy Frozen Mallet, that give life and AD.
Fifth we buy Lich Bane, that give us additional life, AP, MR and an useful Pasive.

Now we have all we need, but if we have more gold, we can buy AP, to kill faster the enemies. (Rabandon Deathcap and Void Staff)

Good Luck ^^

In runes we have MP, helpful to take more damage, cooldownreduction, to do an ability faster and ManReg, to not run out of mana quickly.