How should I start planning a book?

As I writer who has a bunch of ideas coming up to her mind, I don't know how to keep my interest in any of those. After a couple of days, what at first seems like the best idea I've ever had, ends up being a project which is not atractive for me anymore.

I urgently need some tips for being constant with my story. Once I have an idea, what should I do with it? Should I do a brainstorming? Maybe plotting or creating characters?