First of all, find good teachers or mentors who inspire you and make you believe in yourself.

1. Music theory - You need to have good knowledge of music theory. It should be like your second language.

2. Good knowledge of how you can write for different instruments, how they sound, both together with other instruments and alone.

3. Have good technical skills on one instrument yourself, and know how to be a musician in an ensemble.

Finally, you need a lot of imagination and stamina to make it as a composer. Or a lot of luck!