You can get this in the first 5 minutes of the game. After you have entered the first house and got the hand-gun and other gear exit the house and you will see to your right a small building with a car in front (you can not enter this building) it is just down the slope in front of the house if you go right. If you jump on the 45 gallon drums behind the building you can then jump onto the fabric overhang on the side of building (a little tricky a jump but is very doable). Once on the roof you will find a body with backpack and next to him is a BETTER handgun then the first one you got and a SILENCER. Pick them up and you can now equip the silencer to one of your handguns and POP goes the achievement.