How to write a story

I have never written anything but would like to write at least one story. I have tied to start a least 7 good ideas with various beginnings but always change a few details. My mind is willing but the body of my stories are held back for the next step. All stories have at least 3000 words and I have not even begun to write the story.The longest is 17000 words and still I have not even started on the body of the story. I can never decide how to plan my next step to get to the end. The ideas are there but as soon as as I have them a few weeks later someone else has already written it, so decide not to go any feather . Any suggestions on how to lay out a good story to write? so that I can at least do one.
I need coaching.