Back Street Zombie - Print Topic

I like the premise, just a few things that could help you out.
I'd get rid of the second line...

                          All of this stands out amongst all the more traditional garbage normally found in
                          a garage. Boxes. Gardening tools, and other bits of pieces and junk.

...we are already aware of the contents of a typical garage, so the first line is all you need.

The fourth line could be worded better, maybe...

                    Violent banging on the closed double doors grabs Josh's attention?

Try to utilise exclamation marks e.g,

                                                        CALLUM
                                        Back up!

                                                         JOSH
                                        No, don't. Please! I don't want to die!

Read the dialogue out loud with and without exclamation marks and you'll see the difference. Just gives it more impact.

When Josh is bleeding from his neck, his speech pattern would likely be affected...

                                                         JOSH
                                         N-n-no. I-I did what you asked.
                                         This argh! This isn't fair.

I'd get rid of...

               Josh has now completely changed. He's now a
               blood thirsty zombie just like the tied up Barry.

...as you've already stated it previously. You could say...

               Josh loses all the colour in his face.

...and then later on...

               Josh is now a blood thirsty zombie. Just like Barry.

So yeah, just some tweaks here and there to polish it up, otherwise good. Keep at it.