I would prioritate Malady and Hextech Reavolver as important earlygame items.

Start of with "make it rain" as your first spell , an inmportant part of this is the mastery "spellblade" Beacause it deals magic damage in every basic attack , like your "Impure shots" , that is you next spell to unlock.

The most important mastery is spellblade , beacause it deals 5% of you ability power in every basic attack and that makes a great combo with your "make it rain" .