I would like to include the theme of post-colonialism and the implications that European farming practices have caused to the natural landscape (ie. salinity) as well as touching on the issue of farmer suicide.
Sounds good; that should suffice in terms of linking the subject matter, from the sounds of things.
In regards to language features, I was thinking of incorporating gothic elements into my writing and thereby mimicing Kinsella's writing style.
What do you mean by gothic features, exactly? If there are particular ones you've covered in class then definitely go for those.
Basically, I am extremely confused with how to start and how to structure my piece.
If you don't know how to begin, don't begin at the beginning

It's perfectly fine to just start writing stuff that might end up belonging halfway through and then come back to the intro later. If you're still keen on a reflective piece, then you can just write it like you would a normal short story from a character's point of view (i.e. no 'body paragraphs', but different paragraphs for different thoughts, breaking things up wherever necessary). The structure doesn't need to be too complex.