Buy a book off of Amazon. I know that's not free, but you'll benefit greatly from having a properly researched book, and one that you can read anywhere, not just while sitting in front of the computer screen. Keep a pencil handy so you can circle interesting bits so you know where it is when you need to come back to it. Which book? There are plenty. Maybe base your purchase on the reviews and ratings that other buyers have given.
The number one tip I always give people in your situation is to read quality writing. Pick up a newspaper or a leaflet at any time, read a book (that interests you, not necessarily a 'how to write' book), and develop your vocabulary and grammar that way.
A book isn't going to teach you to write. Your just have to write what you like best, the rest will come to you.
I know, who ever learnt anything from a book! That's pretty much it. Glean what you can, follow instructions. Of course not blindly. Read others, and dissect what they've done. And write, write and write some more. Accepting criticism (from those that are qualified to give it) is another step along the path of continuous improvement.
You absolutely have to read quality text. It's almost learning by osmosis. The appreciation and understaning of quality writing leeches into your brain through time.
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