Research the psychology of 10-year-old boys to gain information on their developmental stages, the kind of vocabulary they are comfortable with and the ways they respond to creative input. It helps you get a basic sense of your audience, as well as the tone and pacing you'll need to adopt for your novel.
Determine an appropriate subject for your novel. Ten-year-old boys like exciting stories filled with action and adventure: superhero stories, high fantasy novels like the Chronicles of Narnia, and the like. You can write about whatever you like, of course, but you'll find your readers more engaged in the book if you focus on upbeat adventure stories.
Flesh your subject out into a complete story with a beginning, middle and end. It should have an appealing protagonist, a villain or villains to provide a worthy challenge, interesting supporting characters and a sense of drama that slowly builds towards an appreciable climax. It may help to write up a brief description of each character and a one-page plot synopsis that you can consult as you write.
Set a schedule for writing your novel and stick to it. Writing only works when you keep at it, and a regular schedule can help with that. Pick certain days each week for writing, or make an hour or two every night to write. You may also want to set a word count for each session: write until you meet it.
Continue writing until you have finished a complete draft of your novel. Don't stop until you're done and don't worry about revisions just yet. The important thing is to get it all down on the page.
Let your novel sit for a while---a couple of weeks if you can---and then reread and revise it. Fix any awkward sections, tighten up sentence structure and other grammatical concerns, correct any difficulties with the plot and rework inconsistent characterizations until the book is as good as you can make it. Repeat the process two or three times before moving to the next step. If you have someone you trust, like an editor or an astute spouse, let her read the text and make suggestions on improving it.
Perform one final read-through to correct any spelling or copy-editing mistakes.
Contact a publisher or a print-on-demand service to see about printing up copies of your new novel.