I Need Help With A Shooter!

i am making a game, its very basic all i need is help!
i have not started making it yet exept for the part that i have already designed the zombies and the shooter,

what i need is a code that can make a simple game with a stand still shooter, i need to have zombie walk onto the screen from the other side at random, or all at once it doesnt matter which ever is easier to do. I have the background and stuff, also is there a way that i can get clouds in the background to move?

umm if anyone can help me that would be great also if you would like to see what i am basicaly looking for here is a project game sort of like it:

http://www.bumlee.com/deanimator.html

if you can help me either post the help here or cantact me at dimester3@yahoo.com

if you can help me when i make the game playable i will make sure to put you on the credits as well. so yea thanks alot for all the people that help me, in advance.