It isn't easy to get a job scoring movies. Many of the big movie companies use big-name composers. Some companies have their own in-house composers. The easiest way to get yourself in the running for a soundtrack is to read trade publications and monitor the industry on line, keeping an eye out for independent filmmakers who need music for their productions. Look for student assignments you can work on at any school with a film program. Compose for local theater productions. Do whatever work you need to do to build yourself a list of credits, then get yourself an agent and let him shop you around to the big production companies.
Most movie composers these days work with a computer. There are many music notation and editing programs available. You'll need to decide what works best for you. Finale is one of the leading software programs for music composition. You'll want a computer with enough processing power to handle your software, as well as a music keyboard for composing directly into your computer. Your equipment will also include software that will allow you to view a rough cut of a movie on your computer as you compose.
Writing for soundtracks is different than composing an album that is meant to stand on its own. A soundtrack should, ideally, stand on its own and remain valid long after the movie it was written for has been viewed. The composer should aim for a soundtrack featuring songs that an audience will want to listen to long after they've watched the movie.
But writing soundtracks also includes writing both the mood and filler music for the movie, as well as full songs that help enhance the movie's story. It's a good idea to watch the movie you're composing for a couple of times just to find lyrical or musical ideas. Think about how your songs will help set a mood without disrupting the flow of the movie. Think about the movie's theme or message and write songs that complement those things.