Haha!
A more serious one I would tell myself, is to not allow myself to be a person of convenience. Not to be a person where someone just comes around when it's convenient for them to pop up after weeks, but to have relationships in my life where they make an effort.
I would have also told myself to let myself put people in their place more often in a kind and graceful way. Don't be afraid to stand your ground.
Set standards and expectations. Set a bar. Not just for others, but for yourself. Be dignified and carry some manner of grace. Don't be seen as weak and naive, there are people in low places in life that will pick up on that and take advantage of you for it.
The most important thing I would have warned myself about is that the most people who are charismatic and overly friendly at first can also tend to be the most dangerous and narcissistic later.