There are two kinds of music theory: first, vocabulary and notation, so you can get a grip on the music; second, analysis, so you can theorize about what's happening on a technical level in music. I use both, and studied both for many years from high school to graduate school.
I don't compose using a computer, because it imposes a "machine mentality" on music. It pushes you to write what computers like to execute, which is effortless mechanical repetition, without respect for the "funky" peculiar aspects of real musical instruments played by humans. You can resist the "computer mentality" and use it only for human musical purposes, but its influence is still subliminal and relentless. I admit many composers willingly let computers influence how and what they compose, and they are very successful. I leave it to them. I write by hand, and use the computer only for printing music and certain specific musical tasks.
In musical structure I'm most concerned to create a feeling of voyage, where every moment serves that feeling. By the end I want the listener to feel every moment was relevant to the voyage, and to feel a sense of real completeness, like a "life well lived", a "game well played", and so on. This is different from popular dance music, where the sense of unity and completion is rather guaranteed by the pre-packaged form, single beat, uniform sound, etc. The special excitement of classical music - which is the tradition I work in - is that more disparate or contrasting vectors tend to be involved, so that creating unity and coherence is more challenging, and therefore more energy is contained in the form. Or, to use another analogy: classical-music form is more eventful, like a run through the wilds rather than a well-paved straightaway.
I try to make the parts for all instruments have an interesting role. One reason is that I tend to empathize strongly, so if I imagine a player saying "This is not enjoyable to play", it bothers me. Fortunately, one aspect of what makes a player's part appealing is having enough rests to re-group, so to speak, and be ready for the next passage. So, I can balance my need for instruments to be silent - for musical reasons - with my desire to give them a satisfying part overall, with good things to play, and "good rests", too.
My favorite part is tinkering with the piece when it's almost done. Making big early decisions about what ideas to use and in what order is the high stakes aspect, where you risk including bad ideas or bungling the form. My sense of self-worth is totally bound up in my own estimate of whether I've written a good piece, so it is frankly a difficult thing for me. I'm a perfectionist. And only at the end, when I'm polishing details, do I feel somewhat "safe" and at ease with the composing process.
As for great advice, of course, the best advice is "know what you're doing", which, for instance, means getting educated about exactly what you want to do with composing - to innovate, have fun, make money, garner prestige, or whatever, as such goals are not always obtainable together. Figure out what you do well, also, so you can go with the flow of your talent rather than trying to be something you're not; and finally: seek immersion in your field, especially in your formative years - be where it is happening, where you can learn, and where competition with others gives you ideas, clarity about yourself, and stimulation.
course, everything in life is a source of inspiration, especially music
itself. Hearing Beethoven's Fifth Symphony in grade school inspired
me to become a composer. I was overwhelmed by the grand emotional content
of classical music - it was like the emotions of giants - of truly great
people caring about truly great things - like the vastness of the universe
compared to the smallness of life on Earth! So my greatest inspiration
is what is called "the repertoire" - all the great classical
music. However, I also find inspiration in other music, especially rhythm
& blues and gospel; and in fact I can be inspired by all kinds of
music when it happens to strike me. It's an individual thing, naturally.
Another inspiration for me is challenge and opportunity. For instance,
I wrote a Concerto for Timpani - one of my most successful pieces -
because it was a tremendous challenge (a concerto for timpani - forget
about it! - the instrument's too limited) and a great opportunity (I
could write the world's greatest timpani concerto - very little competition
in that category! - and in fact I believe I did.) Finally, the possibility
of creating something that will change from notes to sound is the composer's
prime challenge and opportunity.