I'd say do tactics, tactics, tactics. It's good to understand a few basic opening principles, but not worth the effort to study openings if you're below 1500. This seems as a good site after a little glance - http://www.logicalchess.com/resources/lessons/tactics/. If you're feeling good, then this site allows you to solve timed problems - http://chess.emrald.net/ - pretty much like game situations. Or get a book on tactics. About openings and endings start worrying when you reach at least 1500-1600.
I hope that helps.