Writing is a more difficult task than neophytes imagine. Getting "A" on your essay in 12th grade English is not comparable, although it's a good start.
You must acquire knowledge of the craft. Learn how to write for a more discriminating audience. Let your characters tell the story, and short paragraphs are more willingly read than long ones are important examples.
Surprisingly, correct language or grammar is not always required. The best illustration of that, for me, is the character Holden Caulfield in J.D. Salinger's epic, Catcher in the Rye. As the narrator of the tome, almost the entire book is told in Holden's voice, which is, for me, a very annoying ultra-typical teen vernacular.
An important element of the craft is getting your facts straight, which requires considerable research. As the author of a story you are expected to be the expert of your tale. If your research is incorrect, it detracts from the reader's concentration and the believability of the story. Luckily, that is made somewhat easier with today's computers.
For the beginner, most writing coaches suggest writing about something you already know. Catcher in the Rye is also a good example of this. Like Holden Caulfield, Salinger went to private schools. And his personality issues revealed in later life suggest the experiences and thoughts of the unhappy teen were likely autobiographical.
There are many other considerations to taking up the craft of writing, not the least of which is getting your work recognized and purchased by the public-at-large. Many writers write for their own gratification. Most, however, prefer to write for the public's enjoyment and the resulting profits. Thus, an author will spend a lot of time marketing his work, often with unsuccessful results.
A professional writer soon learns that the pursuit of the craft demands more talent and effort than merely writing a good story or essay.