I'd work harder in school and take it seriously, wouldn't slack off and drop out like I did this time. I'd take better care of my health and teeth, exercise more and lay off the junk food, and floss religiously. I wouldn't act like a selfish douche and ruin the best relationship I've ever had with the girl I was happier with than any other and could maybe have married and spent my whole life with. I'd spend more time with my grandparents while I still could. I'd try to get along better with my mom and brother so we wouldn't be so distant now. I'd try to navigate life with my abusive dad better than I did this time, and go to therapy from as early as possible in order to cope with it in healthier ways than I did this time. I survived the beatings once, I could survive them again. In general I'd try to be a nicer, better person toward others than I was this time around. I'd study a few languages from a young age when they soak into your brain better. I'd take the time to really live life and enjoy my youth and appreciate it while I can, instead of squandering my teenage years like I did.

The very first thing I'd do is dance with joy at having a second chance and a clean slate. Quietly of course, because waking up my dad at midnight was... inadvisable.