Thesaurus.com is the link I use most, and you probably use it too.
I use its sister, dictionary.com, second most often.
Wordnik.com is a dictionary that provides lots of examples of the word in many sentences. I think the examples could be more varied, but I don't know of any other source like it. I use it when I've read a definition and still don't feel confident about the nuances of the word.
Patricia T. O'Conner's site is loaded with truth about English usage, grammar, spelling, punctuation, and etymology. What's more, you can ask Pat your own question. I say the informal Pat because I met her. She was kind enough to come to my workshop and talk to the kids about etymology.
You could lose yourself for months in Harold Underdown's site for children's book writers.
The Society of Children's Book Writers and Illustrators (SCBWI) welcomes beginners and published writers. It's a great organization. Alas, you have to be at least eighteen to join.
Becoming a Writer by Dorothea Brande. Old-fashioned writing but newfangled ideas.
Bird by Bird by Anne Lamott. This is pretty adult, middle-school level and above.
How to Write a Children's Book and Get It Published by Barbara Seuling. I read this book so many times my eyes made the print pale.
What If? Writing Exercises for Fiction Writers by Anne Bernays and Pamela Painter. Also very adult.
Writing on Both Sides of the Brain by Henriette Anne Klauser. I still go to this book when I'm stuck and desperate.
Garner's Modern American Usage published by Oxford University Press. The information is presented dictionary style. So helpful for the little, crucial things: the difference between lay and lie or the difference I can never get straight, between bring and take.
The Chicago Manual of Style published by the University of Chicago Press. This is a copy editor's bible.
Woe Is I by Patricia T. O'Conner, whose website I mention above, a much slimmer volume than the other two references, but it's handy - and full of silly poems.
Allow me to suggest my book, Writing Magic, and my blog, which you can get to right here on the website by clicking Blog at the top of the page. The blog is a great place to post your writing questions, and I'll be delighted if you sign up to get it regularly.
I can't help. Sorry! But there are a zillion books on the subject. Check them out at your library or local bookstore. And, if you're eighteen or older, be sure to join The Society of Children's Book Writers and Illustrators.