Take out a subscription to a professional writing magazine. Recommend Writers’ News/Writing Magazine and, for women, Mslexia. All kinds of reasons for this but to name two - entering competitions gets you used to having to meet a deadline and you find out more about the writing world, including how other authors work. The latter I always find interesting.
Writing what you want to write
Write what you want to write. Anything else will seem false. Fashions come and go in publishing as in anything else so it’s pointless trying to write in a subject or genre that sells now. By the time you’ve a manuscript ready, the market will be focussed on the next big thing.
Watching out for the sharks
Be aware, as in any industry, there are charlatans. Not only can the writing magazines alert you to these, they can point you to websites like Jonathan Clifford’s anti-vanity press site for further information