Attend a screenwriting class. Many online colleges and universities offer writing and screenwriting courses. Direct instruction can provide you with access to an expert's experience and instruction.
A movie script requires a specific format. Locate online freeware that helps support scriptwriting. Virus-check all freeware before installing it.
Find source material for the movie script online. Hacker and computer-based movie scripts have been made into many popular movies, including the films Hackers, Sneakers, You've Got Mail and more. The information superhighway allows you to interact with others, develop characters based on people met online or through online games.
Read scripts. Purchase script books based on films or television programs that you love. Read as many scripts as you can get your hands on. Like any fiction writer, you want to read as much as you can to see examples of what works, what doesn't and how to manage pacing.
Write, write, write. Write every day. Submit your work to online and offline critique groups. Remember that to make a movie script online or off requires constant practice and development of your writing skills.