Simple achievement, obtainable on the first part of the map you have access to after starting the game "proper".
Make sure you have a car. Now drive around until you see a black rubbish bag - they're all over, outside stores, houses, trailers etc.
Walk up to the bag and press
to pick it up. Drag it to the boot (trunk) of your car, and press the button again to put it inside.
Now drive your car to the very north east point of the island on the top left of the map, to the north of the telephone icon and the time trial icon which are there. You'll know you're in the right place because of the garbage trucks outside the dump.
Drive into the dump site, retrieve the bag from the car by approaching the rear of the vehicle and pressing
, and drop the bag on any of the huge piles of rubbish which litter the place.
Now pat yourself on the bag for doing a good deed and enjoy your achievement!