Begin by opening a new flash document in AS2
Create a drawing of a bullet, select it, press F8, or right-click and select Convert to Symbol, in the dialog box name it bullet and if not already on advanced, select advanced in the bottom right-hand corner of the box. Then select export for actionscript and give it a Linkage name of bullet. For now I'll just use a black circle.

Then draw a circle turret of some sort with a notch or tube pointing to the very right to look like the "barrel" of the turret and make it into a movieclip.

Place the turret in the center of the stage and give it an instance name of 'turret'.

Really firing bullets off is just attaching movieclips to a set x-y position and having them move via this._x+=speed or this._y+=speed.
To do this we will begin by declaring some variables.
Remove the bullet from the stage by selecting it and pressing delete.
Click on the Actions Panel(F9) for the frame.
     We will declare:

var bulletSpeed:Number=5;
var xSpeed:Number=0
var ySpeed:Number=0
var reloadSpeed:Number=100;//milliseconds
var bulletOffset:Number=10;//pixels
var randomNum:Number=0;
var life:Number=0;
var totalLife:Number=50;//frames
var reloaded:Boolean=true;
var x:Number=0;

We use var to make it easier for the computer to know its a variable and :Number or :Boolean to make it easier for the computer to know what type of variable.
These I believe are pretty self-explanatory variables on at least what their role is going to be.
This is what we will be creating;
Use the arrow keys and spacebar to fire.