Note: Written in response to How do I start writing? which was closed as a duplicate of this. Not a question about short stories specifically, but hopefully it will help.
Everyone has different methods for starting, but I’m happy to share mine if it helps.
I’m a plotter. I wrote my first book by ‘pantsing’ (to write without a plan off the seat of your pants) but because the story changed through the life of the novel, those changes rippled all the way back to the beginning and I ended up rewriting the beginning maybe twenty times. It took three years and felt like such a waste of time. So, my second and third novel I have plotted. The story still changes, but much less than just making it up as I go. My second novel took ten weeks to draft by comparison.
So, the first thing I do is pre-writing. I use Scapple and Scrivener. Scapple to plot and Scrivener to write and I find both exceptional tools. I gather photos of what my characters look like, and photos of people I’d like to base them on (I usually composite my characters from people I know well) until these people inhabit my mind on a daily basis. I visit settings (where possible) and take photos and notes. I do lots of research and keep notes. I imagine scenes that I really want in the book and start writing snippets of dialogue. And then I put this whole lot into Scapple. From there, I do a rough outline of the plot.
The next part is by far the hardest for me: starting the book.
I procrastinate for at least two weeks. I sit at my screen, staring at the blank page and check Facebook, Gmail and answer questions on here. Then I chastise myself … A LOT. Every day I try to spend less time procrastinating and at least try to write something, even if it’s just a line or two. And then I chastise myself for only writing a line or two. After beating myself up for a couple of weeks, I find I’m spending less time procrastinating and more time writing.
As @SimonWhite points out, the secret is to write every day, even the weekends, even if it’s just a few hundred words. The reason is that writing a muscle that atrophies without use. The longer you leave it, the harder it is to use. Annie Dillard expresses it perfectly when she says:
A work in progress quickly becomes feral. It reverts to a wild state
overnight. It is barely domesticated, a mustang on which you one day
fastened a halter, but which now you cannot catch. It is a lion you
cage in your study. As the work grows, it gets harder to control; it
is a lion growing in strength. You must visit it every day and
reassert your mastery over it. If you skip a day, you are, quite
rightly, afraid to open the door to its room. You enter its room with
bravura, holding a chair at the thing and shouting, "Simba!” ― Annie
Dillard, The Writing Life
Start with a few lines of dialogue. A short scene. Write something in the middle. Write a scene that’s already clear in your head and is going to be easy. Make starting as easy as you can for yourself and start with small goals. And write every day, Even if it’s awful.
Anne Lammott (Word by Word and Bird by Bird are wonderful) says:
Now, practically even better news than that of short assignments is
the idea of shitty first drafts. All good writers write them. This is
how they end up with good second drafts and terrific third drafts.
People tend to look at successful writers, writers who are getting
their books published and maybe even doing well financially, and think
that they sit down at their desks every morning feeling like a million
dollars, feeling great about who they are and how much talent they
have and what a great story they have to tell; that they take in a few
deep breaths, push back their sleeves, roll their necks a few times to
get all the cricks out, and dive in, typing fully formed passages as
fast as a court reporter. But this is just the fantasy of
the uninitiated.
I also find word counts a huge help and Scrivener handles those brilliantly. I don’t allow myself to get up from my desk until that word count is completed. Without word counts, I wouldn’t have a hope of finishing. So, set yourself a really small word count per day, like 300 words, and write them every day, even if it’s 300 words of rubbish. This is how I start.
Good luck!