1. Talent. You must be born with the talent to compose. Anyone can learn to compose acceptably, and moderately talented people can become better composers, but no one can actually make you a great composer if you don't have the talent to start with.
2. Skill. Composers must acquire every skill possible. There is no such thing as an “unnecessary” skill, and there is no such thing as “unnecessary” knowledge. Performers learn scales and arpeggios, etc., and the equivalents exist for composers. The acquisition of skill never ends. Good composers continue to learn and explore through their entire lives.
3. Passion. No one needs another composer. If you are not driven to be one, quit now. If someone can stop you from composing, you are not a composer. If you compose music only for class assignments while you are in school, you are not a composer. If you are perfectly happy with the music you are writing now and don't feel the need to change, you are not a composer.
4. Love of music. Listen to music. Know some music. Know standard repertoire. Explore unusual repertoire. Perform. Go to concerts. If you hear something you like, sit down at the piano and try to re-create it. Get a score and look at it. Try to figure out how the composer did what he did. Be involved with the music that already exists, because it is your best teacher. Many people, even some professional composers and teachers, insist that traditional repertoire is redundant, and that computers have made training in traditional repertoire unnecessary. This is not true. No great music will ever be written by someone ignorant of tradition. Ever. I guarantee it.
5. Flexibility. Learn to adapt, musically and personally. All great artists change through their careers. Not one great composer worked in one and only one language-- they all grew and re-invented themselves. Don't ever shut out a process or style because you don't “like” it. Every language and style is potentially a resource. You do not know who you are going to be 30 years from now, so you need to build the tools to support whoever that person will be.
6. Flexibility. Take whatever opportunities come your way. If you are flexible in the way item 5 implies, you also need to cultivate professional flexibility. If someone asks you compose Country and Western music for a play or film, do it. If someone asks you to write a ceremonial piece for accordion, bagpipes, and bass drum, do it. You will learn and grow. Do it for free, if necessary.
7. Humility. Great things have come before you. Great people have come before you. Respect them. Never put yourself before the art of music. Never use the art of music to aggrandize yourself. Serve the art-- do not try to make the art serve you.
8. Make an effort to be a complete human being. Art does not grow in a vacuum. The best artists are interested in everything. All the other art forms, religion, science, and philosophy are there to help you grow as a human being. The world is a fascinating place, and human beings are complex and extremely detailed. This is the well-spring of art.
9. Mentorship. No composer achieves anything unless someone believes in them. Select your teachers wisely, out of respect and commitment. Good teachers mentor their students beyond the basic process of instruction. Performers can also be mentors. And when you achieve success, take your responsibility to be a mentor to younger composers very seriously.
10. Luck. There are many people who can do the job-- not everyone who can do the job gets asked to do it.