Once you have completed the game on any difficulty, the "CHEATS" section will be added to the pause and main menus. From there, you can purchase a number of different cheats. If simply want the achievement, the cheapest cheats to buy are any of the unlimited ammo ones, as they cost 5,000 salvage.
Buying or enabling cheats does NOT prevent you from unlocking any achievements. Using cheats is actually a good way to beat the game on the hardest difficulty if you are having problems.