There are a few for whom artistic expression flows naturally.Most of us have to sort of hack away at something and them go over it to smooth it out.
You might try reading descriptions of the "three act play" to get an idea of a story format.
Some just start writing and see where they are inspired to take the characters. Others have an idea of what the story will be and then "flesh it out" in the different parts.
The key is to keep writing. Even it it is painfully bad, it can be made better. A word processor and a recursive approach lets to add, subtract, and move things around. You may wish to dabble with elements of humor, emotional connection, circumstances that will bring out courage, endurance, friendship, or other characteristics.
You want to create characters that your reader will want to invest himself in knowing and feel rewarded for having done so.