I wish I got an internship to help out with research my last year of high school because then I'd have somewhat of a head start in college and maybe I'd be attending my first choice university instead of my second choice.
Wish I'd changed voice teachers maybe. The one I had was not very serious & had me doing lighthearted numbers from musicals, which I loved, but I needed classical training for later in college.
I needed to learn important stuff like like always to sing with your tongue slightly pressed against your bottom teeth (the sound is better), and to sing with a dark sound on the way up of a sequence and to lighten it up on the way down. I did finally get a good voice coach as an adult and still sing semi-pro. Sing in a great chorus / choir even tho I'm middle aged.
Gone to see my guidance counselor more, and followed through on getting my transcripts sent out. My transcripts never made it to the colleges and I ended up starting a semester late.