Well, start with a clear idea of what you want the story to be about. Maybe think of a unique concept or a problem your characters need to solve. Build interesting characters and give them goals and conflicts to drive the plot forward.
To write a great story line, you first need to have a hook that grabs the reader's attention. Then, introduce rising action and complications to keep the tension going. Make sure there's a climax that's exciting and a resolution that leaves the reader satisfied.
A good way to write a story line is to map out the beginning, middle, and end. At the start, introduce the characters and the setting. In the middle, throw in some challenges and twists. And at the end, bring it all together neatly. Also, make the story have a theme or message that gives it depth.