You want to write an adventure story, but you don't quite know what the adventure is. Choosing the style and outline of your story before you even begin the writing process can be a helpful tool. Other things to keep in mind are character development and environment. Determining those might help narrow down what kind of adventure or action could take place in your chosen locale.
Decide what style you want your story to be in, whether script, traditional novel, first-person, third-person or narrated. Aside from a witty one-liner as your opening, your writing style will be one of the first indicators of whether your audience will want to continue reading.
Map out your story before you begin writing. Depending on how you like to write, you may choose a loose storyline to follow rather than one that is highly scripted. Tight outlines allow for little creativity if you find your story unfolding in a different direction. Rather, have your outline consist of only key points in your story, such as the beginning, end, fights, character arch point, deaths and important events as well as a loose timeline.
Create your world. You may want to get out a sketchpad and pencil and begin sketching a map of a continent or country you will be writing about. The world around your characters will flesh out your story and create a believable atmosphere. Decide what you want from the world around your characters, such as different creatures and cultures.
Choose what type of protagonist you would like: male, female, teenager, middle-aged, snarky, happy, angsty, standard heroic, opposed to being a hero, optimist or others. It is especially important for your protagonist to be a three-dimensional character with multiple personality facets. As with all your characters, flesh out an arc for your protagonist. Decide what will they overcome or learn during the course of the story and what their inner conflicts are.
Flesh out your villain, or villains. A good villain can be the highlight of your story. There are many standards for villains, such as villains we love to hate, conflicted, pure evil, villain with a vendetta against the world, against your hero, in love with the heroine or others who make the protagonist's life difficult. Again, create an arch for your villain. A villain who does not think what they're doing is wrong is often more compelling than the "evil-for-no-reason" villains.
Write a compelling love interest. No one wants to read about the love interest that has no personality and no past. Decide what you want out of that character: Do they love the hero, the hero's companion or the villain?
Create companions you can care about. Companions should complement the atmosphere and the other characters in the story. If you can't find a reason for your companions to be with your hero or they lack personality, they probably shouldn't be there in the first place, as adding companions just for the sake of it leads to empty, forgettable characters.