Adventure novels can cover a wide range of era and subject, from westerns to swashbuckling pirate stories to Tolkienesque fantasy to Indiana Jones--style exploration fables. Their common bond is an epic scope, an emphasis on exciting action, and a desire to transport the reader to a faraway time or place. Countless adventure novels have been written over the years, but many of them succumb to poor writing, hackneyed plots and the thousand tiny cuts of stereotyped cliches. If you're going to write an adventure novel, it pays to write one as well as you possibly can.
Define your plot and characters. An adventure novel needs a hero for the audience to follow--someone competent and strong, but possessing a few human foibles to make him or her relatable. He requires a foe to fight (someone evil, but also possessing human qualities to render him accessible), assistants or sidekicks to aid him, obstacles to overcome, and an overarching purpose or quest of monumental import to achieve.
Outline your plot, identifying how it begins, what events take place during the hero's journey, and how (or whether) he obtains his ultimate goal in the finale. The events of the adventure story need to be interconnected and build steadily toward a rousing conclusion. They require internal logic (i.e., there needs to be a reason why they happened) and must flow into each other with a sense of natural rhythm. Furthermore, characters need to undergo a change as well: an "arc" that shows their growth and experience throughout the action (beyond just finding the treasure or saving the girl).
Eliminate cliches in your plot. Elements which have been seen a thousand times before or which derive naturally from a well-known source should be avoided. Work on bringing new twists to your situations: challenges that haven't been appeared in previous novels, new ways of getting out of old situations, character traits that defy the norm.
Write your adventure novel based on your original structure. You need to establish the principle characters early on, define their relative position in the plot (hero, villain, etc.), set up what they stand to gain or lose and describe the events which lead them to the finale. The climax needs to be your showstopper: the "money shot" in movie terms where everything stands to be won or lost. Previous events should build up to that, and you may wish to include a brief denouement afterward to tie up any loose plot threads.
Maintain a brisk tone throughout your novel. Adventure stories thrive on excitement: high speed chases, last-second escapes, desperate plans born of feverish inspiration, and larger-than-life stakes should the hero fail. Although the pace will ebb and flow and there may be some downtime between the hero's activities, make sure you don't get bogged down in excessive detail or ponderous dialogue.
Revise and edit your novel to improve the text. Very few pieces of writing are perfect on the first draft, and a polishing process will help your novel become as good as it can be. You may wish to let a friend or trusted colleague edit it as well: someone familiar with literary structure and who can provide honest, constructive criticism.