Study music theory to learn about the principles of chord construction and progressions. By learning not just what chords are but why they exist is an important part in learning to write your own original music. It is good to know about major, minor, augmented and diminished chords, but the composer who understands how they relate to the overtone series will be able to create new chords.
Studying counterpoint is the next step toward developing a solid understanding of music. Counterpoint is what composers including Haydn and Mahler have studied to learn how independent lines come together to create harmony and melodic lines. Counterpoint will teach composers about proper voice leading and why certain motions are preferable to others. Again, it is more important to be able to understand why something is done than how it is done.
Study musical form to learn about how pieces are put together. Many composers have great ideas, but they do not know how to effectively present them to the public. This is an extremely important subject for any composer, as it will teach them the principles about how music is put together on a large scale. Composers will also learn about the most successful forms in history, such as binary, ternary, sonata, aria and concerto.
Find a private instructor to begin to learn advanced concepts not available in the previous steps. A private instructor will take the knowledge you have learned and help you to mold it into an original and coherent style of music that is all your own. (See References for instructors.)