This is harder than it sounds, because it won't unlock with just any beer from any trashcan (e.g, I found a beer in the trash at the train station, but no luck). This achievement will unlock in the slums. When you first get there (part of the first main quest after the Prologue), get rid of any beer you have in your inventory to make things simpler (just drink it all). Upon enter the slums, if you turn right you'll see a trash can which has a beer in it. Pick it up and drink it and the achievement should unlock.