This will be the very first trophy you get and is completely and totally impossible to miss.
At the start of the game, Lucy will ask you to get into the Animus. When you do, you'll witness the birth of Ezio. His father will then prompt you to hit A to move your legs, B to move your right arm, X to move your left arm and Y to move your head.
Once you've done this, up pops the trophy.