Write an Outline:
Now when I start trying to put a book together I usually grab a note pad and lay out a very basic outline. I have a general story line in mind, or part of a story line, and I write that down. Then I have questions, and I write those down too. By the time I get my outline done, it usually has a lot more than just a plot outline.
For fantasy novels my outline usually has a brief plot outline, a section for main characters, setting, how magic works in this world, a map, and some other information that I might use later. For example, in the outline for Stones of Power The Exile, I wrote out some things about each part of the Kingdom of Denall. The people in the west run the import & export for the kingdom, the people in the eastern mountains are miners, tough and rough around the edges. etc.
The outline and background information helps me add fun details as I go. It is also a great reference in case I forget what color a character's eyes are, or some other small detail about a character. I keep it all in one place as a personal reference.
Have a different hook:
While making a basic plot/setting/character outline it's really important to have a hook. Something that makes your story different and interesting. All stories will have some similar elements, especially among similar books from the same genre, but you need to add something to the book that makes it unique.
For the Stones of Power the gifts people have and the way the gifts make themselves manifest was unique. The actual Stones of Power coming together is a fairly common theme, but how they came together is also fun and different. It was set in a world similar to our own, but with different features, plants and animals. There needs to be enough different that your book doesn't blend in with other books, but enough familiar that the reader isn't completely lost.
While having an outline and a fun plot twist or two is essential parts of a well rounded book, the key to starting writing a book, is to get down and start writing. With all I said about the importance of outlines, I had thirty pages of the Exile done, and a basic idea in my head, before I even thought about an outline. After writing those thirty pages, I learned so much and the entire story changed.
You'll find that the outline changes as you get into the story, and add details. It's actually a fun process to see how your own ideas shift and morph. I used to think it was crazy when people said their characters wanted things to be a certain way, but I find that happening with me from time to time. I sit down and ask myself, "Would Kaz really act like that?" and the scene I had planned goes in a different direction.
So with all the planning and organizing, get out a pen, or open a document, and start writing. Be flexible and see what your ideas become after you start getting them down on paper. Enjoy the ride!