Lessons I have learned as a young composer:
Make sure that you love music, listening to music, performing music, not just composing. If anything, the synthesized sounds that are swirling in the brains of many Finale and Sibelius-driven composers do more harm than good in the compositional process. Know what a beautiful sounding oboe sounds like before you ever write for it. Play piano before you write a piano concerto. Have a jam session with your friends, or sit alone with your major instrument before you start writing anything. Pull out a pencil and sketch ideas before you ever fall subject to the lust of synthesized sounds.
Real music exists for real people. Don't expect that you can write an awesome piece by sitting at your computer for sixteen hours, and then try to find people to play it. Instead, start with the people you are writing it for. Find a middle school band, a small church choir, a community strings group. Don't be pretentious and think that "they aren't good enough for my music." Instead, listen to them. Find out what sounds beautiful for where they are, and who they are. You will find many generous musicians who will perform music for you if you ask. But imagine how meaningful it would be if you wrote a piece just for them. Not only will you have a meaningful compositional opportunity, you will strengthen the bond between composer, conductor, performer, and listener.
Study scores. Don't just listen to recordings. Don't just say "I don't write well for strings." Find a string excerpt you love, read the score; and steal, steal, steal! You can't find your own compositional voice if you don't understand the many successful and visionary compositional voices that came before you. Who is your favorite living composer? Where do they live? Can you take a lesson with them?
Composition is a continuous learning process. Learn from the beautiful sounds and the beautiful people that make them.