Welcome to MR! I'd echo the sentiments of everyone else and put pen to paper.
I'm guessing you have an idea already, but probably arent sure where to begin. I would suggest opening up a blank document and writing the following;
-A couple of sentences summarising your idea.
-What characters you'd like to incorporate and any ideas for Protagonists and Antagonists you already have.
-What kind of setting you'd like the story to take place in. Is it a fantasy world? 18th Century Europe? etc.
-What style of writing you'd like to try and the genre/s you're looking to write for.
-Think about why you want to tell that story? Does it have some important themes or powerful messages running throughout it you want to be present? Or is it a really fun and cool plot you'd like to explore?
This will give you maybe a page or two to hopefully spawn more ideas. Then add to it. Think about where you want the plot to go. Just bullet point down some major set pieces or beats that come to mind you'd want to include. If you're struggling for these, then think about stories in a similar genre or style and which events you really liked. If you loved them, try to figure out why they were important to you. Is there something within them you can incorperate into a narrative yourelf? This will also kick your brain into motion and get you writing. Before you know it, you'll be feeling ready to make a start on writing it.
Lastly, for more technical aspects or specific questions, head to the Writers Discussion Board. Either search for a topic thats suitable or make a new thread in regards to your question and we'd be happy to help