STEP 1: Think of the main characters:
STEP 2: Think about the relationships between characters, e.g.: Are they friends, family, enimies or in a superhero training club together? How did they meet, or did they already know each other? There are endless possibilities, if you are stuck, look at the relationships you have with other people for inspiration.
STEP 3: Think of a setting, is it in the snowy mountains or on a tropical beach? What setting best suits your characters and which environment do you prefer to write about?
STEP 4: Think of a plot; how does your story start, what problems do your characters face, and how do the characters solve the problems? Think of a clear Start, Middle and End to your story. Who is defeated, and who stands victorious?
STEP 5: Work on the details e.g.: How do your characters reach the middle and end of your story? Are there any minor problems that they face on their journey? Do they split up or decide to stick together? The choice is yours.
STEP 6: Now that you have a clear path for your protagonists to follow, bring it all together into a story by using all the planning that you did in the previous steps.