The best advice I can offer people on how to improve their writing is to actually write, and get it finished. Sounds daft but there you go. So many people get bogged down with endless world-building that they never actually start the story, or they agonise over every word and paragraph until the story dies a death. It's supposed to be fun, dammit! Get that rough draft finished and go back and fix it later. So what if it's as wonky as a three-legged donkey - that's what editing and rewriting is for!
The second best advice I can offer is to join a decent writing group that includes people who don't just offer back-patting mindless platitudes, but instead say this does not work for me because of X and Y, but you could try Z, which I think would improve it. Real and constructive advice is gold.
I'm incredibly lucky to be a member of the Glasgow SF Writers' Circle, an offline genre critique group from Glasgow, Scotland. Not everybody is lucky enough to have an established writing group in their area however (never mind genre writing), so your choices are to either start one, or go online. Some of the forums linked at the bottom of this page have places where you can post your work for critique (click here for advice on how to critique) - but please do the same for others and together you can help improve your skills. However, if you chose to do your thing online, be aware that if you post your story in a publicly accessible forum or site then it counts as being published, and you won't then be able to sell your story to professional magazines seeking first publication rights.
Online Writing Resources, or How to Write
Hal Duncan on How to Write a Sentence - Nuts and bolts of taking a sentence of poor prose apart and polishing it until it shines. Excellent examples.
Hal Duncan on How to Write a Paragraph - Same as above, but with whole paragraphs.
Hal Duncan on How to Write Point of View properly
Gary Gibson, on Writing
Chuck Wendig's posts on writing
National Novel Writing Month (nanowrimo) - Runs every November. Join this to kick-start that novel you've been planning on writing for years.
Caro Clarke's Writing Advice
Ursula K. Le Guin’s Essays On Writing
Brent Weeks' Writing Advice
Trudi Canavan Writing Advice
Gareth L Powell: 8 Steps To Becoming A Published Writer
Darren Shan Writing Tips
English Grammar Guide
Guardian & Observer Style Guide
Books on Writing
On Writing - Stephen King
Wonderbook - Jeff VanderMeer
The Penguin Guide to Punctuation - R.L. Trask
Get Started in Writing Young Adult Fiction - Juliet Mushens
The Elements of Style - William Strunk Jr. and E.B. White
I find it useful to read the credits of old TV shows, or you could visit any of the many baby-name sites on the web. I also find sites with lists of historical names very useful to plunder for inspiration: Roman names, Celtic names, etc
Etymology and History of Forenames
Etymology and History of Surnames
Fantasy Name Generator
Cartographers Guild - THE place to go if you want information and tutorials on creating maps for your fiction, or you can commission them.
How to Format Your Manuscript for Submissions
Standard Manuscript Format - but each market may have their own house rules, check before you submit.
SFWA's manuscript preparation
Searching for Markets
The (Submission) Grinder - My preferred site.
Dark Markets (For horror)
Duotrope - Paid service.
The Horror Tree
Publishing - Look Before You Leap!
If you are offered a contract by a publisher, please, please check they are not scammers or a vanity press. You should MAKE money, not pay them for the privilege. Self-publishing is a whole different kettle of fish however, and I'll likely post some links to reputable editors and proofreaders at some point.
Absolute Write Bewares and Reputation Check
Absolute Write publishing boards
SFWA's Writers Beware
Querytracker - A very useful tool for searching for agents who want your particular genre of novel.
SFF World writing forum
Other Useful Stuff
Google Ngram Viewer - When were words common?
Manly Slang From The 19th Century
A Guide to Travel in a Fantasy Setting
Medieval Fighting: A Quick Myth Buster Guide
Lindybeige - loads of videos on all things ancient. From wearing cloaks to torches to weapons and armour.