I'd say working on openings and defences is very important at your/our level.  At least as important as endgame.  Quite a few games are lost because in the first few openings.  Loss of position etc. can handicap you for the rest of the game.  Furthermore, if you are playing games 5 minutes or less you have to know some openings or you'll get handicapped again by the loss of time.  Plus it just adds another dimension to the games.

Scicilian is a real eye opener and a great way to grab control from white playing king's pawn.  Scicilian is kind of rare in this respect as far as defences go.  You can't learn an openign without learning a defence.   Learning openings will teach you how to respond to these openings when you are playing black.

When I started looking at openings/defences I started with these three.

Queen's Pawn Gambit

King's Pawn Gambit

Scicillian Defence

(I really haven't gotten much further than that but I got these down solid)

thanks

At your level, you'd be better served concentrating on the endgame.  That's not always where games are won and lost, but barring any tactical errors in the middlegame, it's almost always why games are won and lost.

However, to answer your question about defenses for Black, it really depends on taste - whether your style of play is based more on tactics and initiative or space and positional considerations.  There are plenty to choose from, ranging from the solid and drawish (like most typical variations of the French) to blazingly sharp (like many variations of the Sicilian).  My best advice would be to experiment and figure out, over the course of many games, what feels most natural to you.  Again, though, while it can't hurt to pick up a little basic opening theory, it would be a better idea to hold off diving into the details until you're more solid on your endgame theory, and can handle the middlegame well enough that you get to play the endgames.  ;-)