if (Greenfoot.isKeyDown("space"))
{
    Bullet b = new Bullet(); //create a new bullet
    getWorld().addObject(b, getX(), getY()); //add the bullet at the position of the shooting object
    b.setRotation(getRotation()); //set the bullet's rotation the the shooter's rotation
    b.move(5); //move it a little bit so the bullet does not start in the middle of the shooter
}

private int shootDelay=20, shootTimer=0;

//...
if (--shootTimer<1 && Greenfoot.isKeyDown("space"))
{
    shootTimer = shootDelay;
    //rest of code above
}