XNA GameStudio 2.0
The instructions for this game are nowhere near complete enough to create a full game. The rest of the work will need to come from you.
Not So Tricky
An obvious feature missing from the game is scoring. The player needs some sort of reward for shooting the enemies. A new field is usually created to store the score. Think about where in the code the collision between an enemey and a bullet is captured. That will be where you increase the score.
You also need to draw the score on the screen. Look back at the first game to see how to do that.
We added sound to the first game. You could do with having some noise when a bullet is launched and when an enemy is killed. You could look up online how to make a sound play continuously and then set up some suitably moody background music. I like to use a sequencer and some samples to make my own music. If you have access to some suitable tools - even if it's just a rubbish ejay package, you can make something orignal. Alternatively, find out how to convert one of your thumping mp3 tunes into a wav and use that.
Better Game Assets
A nice background and some lovely graphics might lift this game out of the doldrums.
Requires More Thought
Let The Enemies Fight Back
There isn't a great deal of challenge for anyone playing this game at the moment. If the enemies could shoot their own bullets at the player, there might be more to the game. You would need to keep track of the number of lives a player has, think about starting and stopping the game when this number runs out. You will also have to think about what makes an enemy shoot - usually a random factor helps.
Things You Can't Shoot
It is a little easier to create a new game object. You can copy the code you have used for the enemies. This time, make it so that shooting the object causes the player to lose points - maybe hear a different sound too.
A Little Trickier
You could fiddle about with the way that the enemies move across the screen. Making them move up and down a little is the simplest alteration you might make. A more sophisticated approach might be to make the follow an arc as they moveacross the screen - or even dive-bomb the player with a life loss being the consequence of a collision between the ship and enemy.