This achievement is for performing your first Take Down.
This is a very easy achievement. In Chapter 2 when you first enter the Sarif Plant, the story will give you a short tutorial on how to perform takedowns.
Takedowns cost 1 enery bar, but if your on your last energy bar, you can wait for it to recharge.
Move up to the first patrolling enemy and do either a Non-Lethal take down (tap B) or do a Lethal takedown (hold B). If you perform the Lethal takedown, the noise will attract the attention of the other guards. Also, performing non-lethal takedowns net your 20 more xp than a lethal takedown.
Easily done and the achievement should be yours.