Listen to music, study the greats and learn all the music theory you can.
Also, practice composing. If you learn something new, try to write something using that new technique. Force yourself to write something, its the only way to learn it. Improvise in your instrument, try to come up with melodies, sing and play the improvised melodies at the same time. Learn techniques how to create variations on melody, in your themes, find out way to recycle themes/phrases and use them later in your composition, learn how to enrich harmony, learn about form, the I-V-I poleralisation. Learn modulations, learn how to use non-diatonic notes and chords, learn about prolongation, learn cadences, learn about the instruments you are writing for, maybe learn orchestration, make sure you learn the language of tonal harmony, try to think creatively, experiment, try to reach the goals and accomplish the musical effects you have in mind.
You see, lots to learn. And different ways to learn. You can read a boring book, you can study your favorite works and really steal from them, you can just try and reinvent or even invent some techniques yourself. Of course you need to use all three of these ways to learn.
BTW, I use my computer to write music, works really well.