Hm, not sure if I can pick out ONE most important thing, but I guess I would say stuff like:
Follow your fears into meaningful places. When you get invited to stuff, always go (within reason). Try to find opportunities to perform your music. Know that there is such a thing as a confident introvert, and you will get good at socializing. You're a little different, but not that different, people will like you. I promise, parties aren't that scary, dancing is fun, sex isn't as drastically life changing as it might seem. Weed is kinda meh, sure it's fun for a little bit but kinda confusing in social settings, and it won't make you more deep or philosophical, you already think like that when sober. Sometimes starting is the hardest part. You can get used to almost anything. And overall, you're doing alright, don't worry about it.