General as Ezreal you'll want to start your Doran's blade+potion and then on your first back idealistically pick up a tear and Sheen. If you can only afford one and feel like you need more damage to pressure your lane, I would take the sheen over the tear even though it does delay your build a bit.
As soon as you have a tear with a sheen, I would try and finish my Iceborn gauntlet ASAP because it provides cooldown, mana, damage, armour, a slow on your Q which ezreal really synergises with. [Don't forget your fancy lucidity boots for the CDR] ;)
Then I would look at my enemies composition.
over
is an option but I feel the slow works a lot better for Ezreal and his team.
Do you see tanks or HP sponges? Build
( I build this anyway if I have nothing else to prioritise)
Is one of the tanks snowballing and stcnking armour? build either
or 
see a lot of CC problems? Rush
into 