To start off with, create a basic projectile. Square are best until you get used to this; the hitTest determines what area counts as collision by an objects bounding box, and irregular objects will still have a square bounding boxso they may seem to collide before the projectile actually enters the visible area of the object.
So: projectile. A small square, which you can then turn into a movie clip symbol. Give it an instance namesomething simple and easily identifiable, like projectile.