I won't say I'm not a good writer, because I'd be lying to myself. I am good at putting thoughts, emotions, and ideas into words that my readers can connect with, and I'm fairly decent at spelling, grammar, and word usage in fiction and non-fiction. But I know I can become a better writer than I am now.
The most important thing that made me a good writer and will keep me improving, to quote James A. MacDonald (co-author of the MageWorlds books with his wife, Debra Doyle), is: BICHOK.
"Butt In Chair, Hands On Keyboard" Or pen/pencil on paper, if that is your preference.
It's the physical act of writing, coupled with a sincere desire to improve upon past performance, and willingness to listen to and consider advice, comments and ideas from other writers and readers whom you respect, that helps us to become "Good". Note that I didn't say to automatically take that advice.
For a long time, I tried to take all of things people were telling me to do and changed my words, characterization, plotting, etc., and it took all of the joy right out of the act of creation.
Suddenly, I wasn't writing
my stories, I was writing theirs, and I hated it. But I was a "dont rock the boat" person and it took a lot of courage to finally be able to say, "That's an interesting point, but I've decided that I'm going to keep my characterization of Aragorn/my description of Alqualonde/my interpretation of the people of the Mark, etc., the way it is."
It was hard, at first--and even now, sometimes--to have faith in my own "voice" and to hold to that despite people pushing the fashion of "single POV" or "jump immediately into the action", when my writing voice and style are not necessarily like that. I've been told outright by a certain author elsewhere online that I write Aragorn "wrong". Another person said that I needed to rewrite a story that I created as an experiment in description to put "action" into it or no one would read it. Another has told me that no one will read my work unless I spice it up with smut.
I tend to write long, descriptive sentences--much like those in THE SILMARILLION, now that I think of it!--and I am willing to write various interpretations of characters depending on how I feel or what the plotline is. I do not have a personal "fanon" in which I force all of my various writings, so from one tale to the next, my characterization will be different (although I work hard to keep that characterization consistent within the story). I like to experiment with "what ifs" and alternate interpretations. I am not comfortable writing smut, so any romantic scenes that I write will be rather chaste compared to the majority of the stuff out in most of the archives.
I'm willing to try out the advice I receive, but in the end it is my choice as which bits I'll take on and apply.
I suppose that what I'm really trying to say is to write, write, write, whether it be just for yourself, for the fanfic sites, or for professional publication.
-- Experiment to see if the styles and techniques that you see other writers using might work for what you write, but don't think that you
must do it their way.
-- Do learn spelling, punctuation, and grammar rules so that when you do break them, you do so knowingly by choice, not from carelessness or ignorance.
-- If you are not happy with something you wrote, consider it, and try alternate approaches if you wish. Some of my best work came from glaring at a piece that just wasn't working and then turning it upside down, or changing perspective, or even changing all of the characters who were involved in the plotline.
Writer's retreats, workshops and conferences can really recharge your muse and give you a lot of information on different ways to write. The books that have been mentioned are useful too, but I think the thing that has helped me most is simply living my life. The 50 years I've been on this planet have given me amazing experiences that fuel my writing. Things like learning to drive a car, going to college, enlisting in the Navy, dating, getting married, having kids, coping with miscarriage, having a congenitally handicapped child, working various jobs, walking around the neighborhood, putting my hands into compost and soil to plant vegetables, learning how to make a sourdough culture and the bread that results from it, burying a parent, holding the hand of a seriously injured child in a pediatric ICU, teaching Sunday school, playing basketball really badly, painting a room, remodeling a house all find their way into my stories somehow, even though 90% of them have nothing to do with Middle-earth.
Yikes, I had not meant to blather on so long!
In short, to be a good writer, write as much as possible, as often as possible, even if it never gets posted; consider others' advice and use it as you find need and desire; and put life itself into your writing.