What are the best and worst choices you can make as you start your career? Here are my suggestions for both.
Best: Good writing teachers. Many of them are found online. A hands-on teacher can teach specific craft skills and can hone your craft far faster than plugging along by yourself. If they were available back in ancient times before the Internet, I could have cut over 10 years from my writing journey.
Second Best: Learning about the business side of writing so you can move forward safely in this sea of piranhas. I recommend Kristine Kathryn Rusch's "Business Musings."
I've not been a member for years, but RWA, SFWA, and MWA used to offer lots of great info on the business aspects of a career. Ask around to see if they still do.
Worst: Self-publishing before your craft is competent. If that first and second book are dreck, no one will buy the next book. The rush of self-publishing also blinds some writers to the need to keep learning craft so they don't bother to keep learning and continue to publish dreck.
Second Worst: Being so eager to publish that you hurt or end your career by picking the wrong agent or publisher, then signing a contract that will destroy your future. Also, don’t throw all your creative eggs into one media aggregator like Amazon Kindle who can casually destroy your career with a software algorithm glitch. Business knowledge is power, folks.