All writers and illustrators should join.. the Society of Children’s Books of Writers and Illustrators
If you are just starting to write picture books, I would highly recommend: Picture Book Magic by Susanna Leonard Hill, Just Write for Kids by Emma Walton Hamilton.
A go-to site for writing, illustrating and publishing Children’s Books is Purple Crayon by Harold Underdown.
A fabulous site for all children’s writing is www.kidlit411.com run by Elaine Kealey and Sylvia Lu, and don’t forget to join their Facebook Page.
If into Non fiction picture books then take a look at Kristen Fullton’s site – Explore Nonfiction.
A great Young Adult literature site is – http://www.adventuresinyapublishing.com/.
For all things about diversity in children’s literature, go to – WeNeedDiverseBooks.com
Querytracker.net is a great tool when you start querying.
Always research agents, editors, publishers, awards, competitions etc well before signing any contracts or submitting your work. Check out Preditors & Editors and Writer Beware.
If you are looking for professional editing, then look no further than editor Emma D Dryden of drydenbooks, bestselling author Emma Walton Hamilton, and author and editor Harold Underdown and author and editor Lorin Oberweger at Free Expressions.
Josh Funk’s Guide to Writing Picture Books… https://www.joshfunkbooks.com/resources-for-writers