Qualifications
You'll need to know how to read, write, and play music. That means years of honing your talents as a musician. Sure you can rely on using an app, but how are you going to play anything if you can't read music?
Piano skills are essentially a requirement, but knowing how to play multiple instruments is a big plus.
Be comfortable with technology. Expert computer skills are a must, especially with Pro Tools and other music composing and mixing software.
It won't hurt to major in music at a college that offers a program in film scoring and composing.
You'll also need to understand both music and the medium you're writing for, such as film, TV, or video games (ideally all three).