You will gradually unlock this trophy after many hours of exploration. If you are like me, you will spend 80% of your time wandering the wastes and the other 20% will be actually doing quests. There will be little triangles on your compass which will point you in the direction of a new place to discover. If this triagngle is filled in however, then that means you have already discovered it.
At level 20 there is a perk called Explorer. What it does is it will unveil all the locations on your Pip-Boy map. You still have to travel to the locations to actually discover them but this can be helpful if you need a few more.