Watch sci-fi and/or action films and take notes on both the positive and negative aspects of each. Study successful films to identify what made them so, as well as movies that flopped to see what they were lacking.
Read information on the technicalities of formatting and submitting screenplays in order to prepare yourself for the process. Writing a screenplay is just as complex as writing a novel and must be approached with the right mindset. A few helpful websites include Screenwriting and Screenwriter's Utopia.
Check your local video store, bookstore and library for films and books (which can be optioned to become films) for any sci-fi stories that sound similar to the one you want to write. Many sci-fi action films use the same tropes, such as a galaxy in danger or an alien attack, which is acceptable. However, you will need to identify these ahead of time in order to make your screenplay stand out in a vast genre.
Write a draft describing the world in your sci-fi action movie. World-building is a huge part of science fiction, as it nearly always takes place in a futuristic or otherwise different society, and therefore the world itself, from larger issues like the government and laws to smaller issues like how characters communicate, requires lots of thought.
Write a sheet of personal information for each of your main characters. Too often, sci-fi action movies focus more on the world, and not on the characters. However, the characters will draw an audience into the story and make them care. Be sure your characters are real, believable and sympathetic.
Write out the plot of your movie. Begin with the three main plot points; the event in the beginning that acts as a catalyst to get the story started, the event in the middle in which tables are turned or a secret or truth is uncovered, and the climax at the end in which all that is at risk is put on the line, and resolved.
Start a new word document with a header in 12-point Courier font that includes the name of your sci-fi movie and the date of the most recent revision.
Write a heading for each scene in all caps. Follow it with "INT" or "EXT," which stand for interior or exterior, depending on whether your scene takes place indoors or outdoors. Also include the exact location and the time of day your scene takes place in all caps, such as "BEDROOM - MORNING." Type the name of each character that appears in the screen centered, with their dialogue and the action events left-justified.
Fill in the dialogue and plot action, using present tense ("is" instead of "was") for the action. Use active voice ("The alien falls to the ground") rather than passive voice ("The alien is hit and falls"). Avoid lengthy descriptions of the technology and world, as the characters and action are what sell a sci-fi screenplay.