Creature cards not on the battlefield and creature spells are not affected.

You must choose an existing _Magic_ creature type.

Replacement effects that modify creatures of a certain type as they enter the battlefield will apply (or not apply) after you apply this effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon, a Runeclaw Bear would enter the battlefield with an additional +1/+1 counter.