Q: What is the most important thing I should know about writing?
A: I'm betting Jenny will have the same answer, but honest, I came up with this on my own.
No, really. I did.
Read good books. And by good books I mean meaty classics, or as Jenny says, "dead people". In my experience, like it or not you will write what you read. So if you want to write a good book, read some. Learn the richness of the English language from authors whose works have stood the test of time. Shakespeare, Austen, Christie, Sayers, Lewis, G. A. Henty (track him down if you haven't heard of him)... Real authors, if I may be so snooty-sounding. That's not to say you can't read current works. Not at all. But if you want to write lasting books, read lasting books.
Books like Harry Potter and Twilight are extremely popular now. But do you know who Mrs. Radcliffe is? She was an extremely popular author during Jane Austen's life, yet I doubt you've heard of her beyond a few mentions in "Emma" and "Northanger Abbey". That a book is popular now does not mean it will continue so.
Choose your materials carefully. "What goes in must come out", as an old song says.