This might be controversial, but for me personally, I would tell myself to not be so afraid to try HRT, that I’m going to take it later anyway, and that I would regret not starting earlier.
I ended up getting HRT off the Internet at 21, but I knew about DIY HRT, and was tempted to buy it, back when I was around 16 years old.
I think wearing a trans pride pin whenever appropriate and safe is a great way to make other trans people more comfortable around you. As a trans woman who sometimes blends in as a cis person, I always treat trans pride pins as a conversation starter.