RULE NUMBER 1: pay attention to your Mana and the moves of your enemy ADC. If he comes to close, try to shoot him and leave his range without taking damage.

RULE NUMBER 2: USE SMARTCAST. It´s really really awsome :D Of corse it´s a little bit difficult at first, but it worth.

RULE NUMBER 3: Don´t use your E to start the fight. Use it only to escape or to kill out of range enemies with low health.

RULE NUMBER 4: Chase at first only the enemy ADC in fights. Use every chance to poke the health of your opponent down. Try to damage him without getting damaged.

RULE NUMBER 5: Use Q in chase of emergancy. Your Q reduces the cooldown of your E by 1 sec every time you hit. It worth.

RULE NUMBER 6: In every fight use your W to reduce your opponent damage per second. It reduces his attack speed and increase yours, because of Ezreals passive.

RULE NUMBER 7: Use your Hero Ability R for this reasons.
-low life Champs. of corse
-defend Tower from Minions.
-steal Baron.
-and in every fight at first. Passive if you know what i mean ;)