My advices:
1. Have some images in the sprite, it will become animation. Assign it as the main sprite of the zombie. It's the easiest way, but it's far from good.
2. The best way is to use "step torwards" command block in Drag'n'Drop or the function which does that; put it in the ste event of zombie object
3. Your character has to have the health variable. There's built in one called "health" (what kind of magic is that
). Then the easy way is to make collision event with zombie which makes the health to be relatively set to -5 or so
4. Unless you are using meele weapons, the easiest way is to have a bullet object, which on create event points torwards the mouse pointer and has speed set also in this direction. There are blocky functions for that, but you can also code it. The shooting can be achieved i many, many aproaches, i recommend this: create a variable CanShoot equal to 1 on the Create Event. Make the global mouse left pressed event, on which if CanShoot is 1, the bullet is created (and it's code makes it fired), CanShoot is set to 0, and one of the Alarms is set to some value, let's say 15. Then, on that's alarms event, make CanShoot to be 1 again.
5. The easy way to do this is to do Destroy event for zombies in which they will create new instances, for example in coords x=-32, y=random(room_height). This will make them spawn only from the left, but it can be upgraded with moderate effort
6. Make something like the main control object that just sits in the room and doesn't have a sprite. If you're going to use spawining mechanic from above, then you'll have enough zombies, and to slowly increase their number, just make an instance randomly pop now and then using alarms for example
Ask anything if you need.
There will be potentially many problems you've overlooked, but i won't spoil you the fun of learning the hard way