Know that it helps to have some knowledge of music even if that means you just enjoy listening to it. Learn how to read music notes if you have a chance-it won't hurt to know that information.
Know also that if you play piano or a music keyboard, or at least know how to use a computer keyboard, that it will be a bit easier to use current music technology.
Check out various types of music composition software. Shop around since software varies in price and complexity. Find software that's user-friendly, especially if you don't have much experience with computers or music.
Find music software at local electronic stores, computer stores and some music stores. Ask store associates for their recommendations. Look for music software online too and read software reviews.
Purchase music software and install it on your computer. Read the instructions carefully ahead of time as some music software requires installation of or connection to new hardware or additional software.
Practice using your music software once you have everything up and running. Compose simple pieces at first to get used to software functionality and sounds before you write a movie soundtrack.
Figure out which movie you'd like to write a movie soundtrack for (unless you have one in mind already). Listen to the movie's current soundtrack, if it has one already, or listen to movie soundtracks from movies similar to yours. Get some idea of how the music should flow and segue.
Pay special attention to the various types of music used for different scenes and events occurring in the movie. Think about how movie soundtrack music matches what's happening in each scene-mood, character, setting and action.
Begin composing. Organize music pieces in sections to write a movie soundtrack. For instance, divide your music by scenes or categorize according to scene type. Know that you can use similar music for similar scenes or you can even use the same music more than once.