If you are a writer (child, teen, or adult) and just getting started, my advice to you is start reading, a lot. There is much to know about your chosen craft, and much to know about how to get your work published. Here are some resources that might help:
A professional, international organization with chapters across the country and throughout the world.
By children’s writing guru Harold Underdown, a valuable resource for both craft and business tips.
My agency’s website. Follow this and other agency websites to start getting an idea of what they like, who they represent and what they’re looking for in their authors.
A website chock full of helpful information. These folks also produce the Children’s Writers and Illustrators Market – considered the Bible of children’s writing, giving you insight into publishers, markets, agents, etc.
For Kids:Stone Soup: I highly recommend this magazine by young writers and artists. It's a wonderful place for kids ages 13 and younger to submit their work.
A critique group is a great way to improve your writing. Finding the right one may take some time – but be patient, and keep trying to find the right fit. My critique group calls itself Will Write for Cake. We love to celebrate good news (any news, really) with sweet treats, but most importantly, we support each other and care about improving each other’s writing. You can meet quite a few of the WWFC crew on their websites: LauraEdge, Doris Fisher, Lynne Kelly, and Miriam King. They have all written some marvelous books.
My last bit of advice: Sit down and write. If you want to write a book, you must make the time and do it. You must be prepared to write some really bad stuff. You must be prepared for rejection. You must be prepared for frustration. You must get off the internet, put down the cell phone, and write.
Unless you sit down and start typing the words, you will never know how amazing it feels to say 'I wrote this, and I like it!'. I encourage you to seek out a supportive network, educate yourself on both the craft of writing and the business of it -- then sit down and write the book that’s in your heart! It will be worth all the hard work.