You get scrap from killing enemies. You get extra scrap if you use a finishing move. Also, the more enemies you kill without visiting the med station, the higher the scrap multiplier. I believe the multiplier goes as high as 3x so go on a killing spree.
You find scrap caches with extra scrap arond. The scrap in the caches doesn't count towards this. YOu have to manually open the caches from your inventory. If you disassemble the caches in the med station, it won't count (the scrap will be banked, you will not be carrying it)
To get the achievement you need to have 100,000 scrap on you.
You will get this naturally at the beginning of NG+, since you get about 150K for the final boss.