I would tell my younger self to get a good editor and not to be pushed into publishing until I was ready too. I didn't promote my first book for a while as I found fault with it. Once it had been edited I felt happy to promote.
I began writing in my teenage years but only seriously back in about 2012 and published my first book in 2013. I had been thinking about a story for a long while and finally decided to write it down. I wrote in secret originally and kept the writing hidden in the bottom of a chest of drawers. After a discussion with some friends on Twitter, my friends persuaded me to write it up and publish and I have never looked back since.