Scratch

The enemy AI could be somewhat tricky, of course it depends on how you want it.
You could make a sprite which moves 1 step all the time and points to the main character.

For the shooting you need a sprites which stays hidden until spacebar is down.
Then it goes to the charcater points in the same direction as it and then you use the repeat until script for the movement.

Power ups could be sprites which changes a variable when they are touched by the main character.
The variables could be for the speed of the bullet (the shooting one)

Health packs are made in the same ways as the power ups but the change a health variable by a certain amount when they are touched.

Ammo packs are the same.

Different guns
Use variables like:
Set Bullet Costume to (GunVariable)

Different Zombies, I think you could figure out that on your own 

Solid walls isn't the easiest thing to make, search for sensors or hitsensors in the forums to learn more.

Damage from the Zombies: When you touch the zombies the health variable changes by a certain negative amount.

I cant wait!

technoguyx wrote:

Well, what you want us to explain is pretty much the whole game except the menus and stuff.

The most complex thing, as you can guess, is the AI. It depends on the kind of AI you want, the simplest AI would be one that just moves randomly and shoots to the player.

Shooting can be done with a bullet sprite with this script:

[blocks]<when green flag clicked>
<hide>
<forever>
<wait until><mouse down?>
<point towards( mouse-pointer
<show>
<repeat until><< <touching[ edge  <or> <touching[ enemy >>
<move( 35 )steps> --Can be changed
<end>
<end>
[/blocks]

Great Cheddargirl! The part of that was really nasty. was'nt it?