The biggest problem with most short stories is that they start too far back. More often than not, any short story by a beginner can be greatly improved by cutting away the first page, or even page and a half. Think about where the story really starts, right in the meat of the action or conflict. You can seed any backstory sparingly throughout as you move forward from there.
The real skill in short story writing is editing. No matter how good you think your story is, it can be shorter, tighter, more compelling. Learn to strip the fat relentlessly, hunt out and kill repetition of words, make every sentence sing. Flense your work to within an inch of its life before sending it out into the world.
Don’t try to write for a market. This applies to novels too, but it bears remembering. You need to write the stories you want to write, the stories you want to read. Tell the stories only you can tell in the way only you can tell them, then go looking for markets to sell them. There are so many online and print magazines and anthologies these days, that you’ll surely find somewhere eventually if the story is good enough.