1. Write and polish a short story.
2. Research the short story market.
3.Make a list of appropriate markets, and begin a submissions record.
4. Submit the story to the first market on the list, and keep submitting.
Write a short story. Really, it's the most important part. So...what's a short story? In a general sense, "short story" means prose fiction that's shorter than a novel. But in that wide range of word count, there are a number of divisions:
Flash Fiction: Usually defined as less than 1,000 words
Short Story: less than 7,500 words
Novelette: between 7,500 and 17,499 words;
Novella: between 17,500 and 39,999 words
There are markets for all of these lengths of fictions, though it's generally easier to find markets for the shorter lengths. Back in the days of typewriters, there was a complicated process for estimating the word count of a story, but today the word count generated by whatever word processor you use works great.
But first drafts are rarely ready to send out as-is. There's a multitude of advice on how to polish a short story, and plenty of disagreement. I'm a big fan of critiquing and blogged at some length about it last year (Parts 1, 2, 3, 4, 5, 6).
Why not just throw the first draft out and hope it sticks? Unless a market specifically asks for a rewrite, they don't want to see any given story twice. If you submit a sloppy rendition, you don't get another chance. Also, if you're trying to use editors as first readers, you're likely to re-revise the story after every rejection and waste a lot of time.