Nice effort for your first game, i like the concept. It's fun to play but also very hard right from the start. It would be nice if the game started easier and progressively gets harder while playing. Maybe make the missiles shorter at the beginning and make them longer after reaching a certain score (every 10 or 20 points or so). You could also use the playfield as a barrier and the ball as enemy later in the game.
Keep up the good work!