Where to ask for advice on book?

You can post an idea for feedback in The Sandbox forum, but I can tell you right now what response you will get, because people ask this type of question over and over.

(1) There are no bad ideas. It's all in the execution. So sit down and write the thing.

(2) Ideas are a dime a dozen. What matters is what you do with it. So sit down and write the thing.

(3) An idea is only "good" if it inspires you to write the story. So what seems like a "bad" idea to someone else is irrelevant. If it interests and engages you, it's a "good" idea. Sit down and write the thing.

So perhaps I've saved you a little time, here.

It helps if you have a specific question (does this seem realistic? Would a 12 year old boy do that do you think? Do I need to research X? Can anyone suggest other books with similar themes I might read? Can you think of a way to strengthen the plot/add a twist? Is my doodad showing? ) rather than just 'is this any good'.

Story Research for when there is a specific thing you need to know (and hunting through the posts there can be a useful endeavor in and of itself).

The Sandbox (password = vista) is for more general questions, though I suggest you should try and get at least something on paper (or typed) before venturing there - as others have pointed out, there is no such thing as a bad idea, it is all in the writing.

Good luck.