If you want to write stories just as a form of expression or a hobby, just start writing what's in your head. Then show it to one or two close friends (it'd be better if they also write or an avid reader) whose opinion you trust. Ask them to be gentle with their criticism and see where you can improve. Showing our writing to someone else is like showing a part of ourselves, so in that sense, it puts us in a vulnerable position, so I know how scary it could be. That's why I think it helps to show your work to someone you know and trust first before throwing it into the wild that is the internet. :')

Reading a lot also helps to get a good sense of story structure and pacing. To be a good writer, it's important to "feed" ourselves with different types of stories and writing to enrich our vocabs and prose, which then will help us to develop our writing style. :)

Hope that helps! If you have any follow up questions, I'd be happy to answer them!