There is so much advice and information out there right now related to writing that much of it is just plain crap advice. It's really easy to give out advice. Advice is an opinion or something learned from experience... but it's from that one person's experience. Not everyone is going to learn to write, or come to writing, the same way and - I'd hate to say it - even my own experience and advice could be crap advice for you.
If it's not crap advice, great, I'm glad I helped.
But please keep in mind that I, too, was shaped by my own path through life and writing. My experiences are probably not the same as yours. Heck, my dialect of English (which is Canadian, not UK or American) is probably different than yours which is why I seem to spell things a bit differently or express things a bit differently.
It's just that my microcosm of experience is not the same as yours. And that's fine. We're not clones. We should be different and we should, within reason, express things differently. It'd be really bloody boring if we didn't have something to make us unique.
With that being said, let me tell you the two worst pieces of advice I ever received in my life:
1) You should pay attention to trends because that's what sells.
Whoa, wait a second here. Now, I'm not saying you shouldn't be aware of them. In that I can agree with the person who handed out that advice. But by the time you finish that book, even just the first draft, the trend you're writing for may already be on the way out.
Take zombies for example. Ever wonder why I have never written a zombie book or script?
Because they're overdone. By the time I finished one, the trend would be over.
And I don't like zombies anyway.
So, following a trend in this case would only bite me in the proverbial ass because my readers (probably you) would pick up on the fact that I wasn't really into it. It'd fall flat... and be out behind the eight ball in publishing trends.
This is what I'd suggest; instead of writing to meet trends, write what you like to read. Write what you're passionate about and put everything you've got into it. Edit it, revise it, rewrite it. Get that manuscript to the best it can be, and then write something else. Keep writing and eventually things come around again. Trends are like that. Look at bell bottoms from the 70's and then the boot cut of the 2010's if you don't believe me. If you submit just as that subject is trending, you're ahead of the game because you're already prepared for it. You may only have to do a few tweaks to bring it up to snuff but you're not working from scratch.
2) Write what you know.
Listen, if we all just 'wrote what we knew', there wouldn't be anything new written. Ninety percent of writing is research and learning new things in life and bring your spin on it.
I live in Northern Ontario, Canada. Now, before the advent of the internet (or, at least, before broadband was a thing), I had a somewhat limited view of things. All I had was the local library, which was pretty decent if a bit outdated, and maybe twenty channels on TV and most of it was just local stuff. That was when I was in the city. If I went to camp, it was even worse. I had three channels in black and white, no internet, no phone, and the library was even tinier.
What I knew was limited indeed.
Don't get me wrong, I went through school and graduated out of high school, but the greater world view (unless we had an exchange student) wasn't there like it is now. There was no FaceBook, no Twitter, no MySpace, no email. No Reddit. Nothing like what we have now.
My favourite example to pull in here is Jessica Fletcher from Murder She Wrote - that was my life back then... minus the murder mystery every week.
I even was limited to what I could write by hand or type on a typewriter, when I could afford the paper and ink reels.
So, how did I manage to write science fiction?
Well, much of that was with what I could learn independently from books from the library, documentaries on tape, and (thank you to whoever approved of building this - you're a literal life saver) Science North in Sudbury, Ontario, and other science fiction shows and books like Star Trek, things written by Elizabeth Moon, Anne McCaffrey... others...
I could go into how computers and the internet changed my life, but that's the subject of another blog post.
But, I think you can get the gist of why these two pieces of writing advice - while they would work for others - were the worst advice I had ever received.