I would have told myself not to study so much and to meet more girls.
Although I'm only 19, there are a ton of things I wish I could go back and talk to past me. Too many times have I missed out on opportunities and lost advantages.
Ah well.
I'd have recognized the beauty of mathematics earlier so that I could be doing cooler stuff now.
I would move my ass to a first world country before I ran out of money to do so.