This is the very first achievement you'll get, and there is no way you can miss it.
At the very beginning of the game, Lucy will ask you to get into the animus, so go do so. You will now watch the birth of Ezio, and once he is held by his *father, you will have to press A to move your legs, B to move your arms, X to move your right arm, and Y to move your head. Once you are done with these simple tasks, you will unlock this achievement.
*It might not be his father, I didn't hear what he said at first.