This video shows how to fix a shelf to a plasterboard wall. If possible it is best to always try and fix to one of the studs, but when that is not possible you can fix to plasterboard as long as you chose the correct type of fixing. If you are not in the UK, this video shows how to fix a shelf to drywall using snaptoggles (which are widely available). Buy snaptoggles here- http://fave.co/1Ozdquh