Okay, where do we start.
Well, I'm not going to patronise anyone and go through the 'first you need the idea' spiel. I assume, by the very fact you are reading this, you already have the type of book you want to write in mind.
So, assuming this is the case, you need to get it down on (electronic) paper. Now I am no expert, and there are countless pages out there advising you how to do just that, but I can give you one piece of advice that many seem to skip over, and that is Just write.
Forget structure, and formatting. Don't worry about spelling or grammar, that can come later. Get your work down on some sort of media, preferably a word processor.
Now that may sound simplistic, but take it from me, a couple of years ago, I wasted no end of time trying to set out the perfect book.. My pages were numbered and formatted beautifully. My spelling was perfect and grammar was great. After 3 months I had written 1, albeit perfect, chapter but had lost my enthusiasm. This was too much like hard work and my story was still untold. I had focussed so much on doing it right, I had lost sight of what was important, the story. I decided to forget all the well meaning, and sometimes expensive advice and get my story down. Three months later, I had a draft novel consisting of 120,000 words sat on my desktop.
So, don't wait. Set yourself a target. A thousand words a day is achievable, but dont stress about it. Do what you can but for heavens sake get it down while the ideas are there. Don't worry if it is an end or middle chapter before the beginning is done, you can knit it all together later.
Don't even worry if you have an idea, but it doesnt fit into the theme of the book, get it down. It may come in useful later, (Or in a different book) That's the beauty of word processing. Thank heavens for cut and paste.
I call it the Rolf Harris approach. For those that don't know him, Rolf Harris is an Australian entertainer/singer/artist and does not write as far as I am aware. But what he is partly famous for is the catchphrase, 'Can you tell what it is yet?'
This came from the days when, as a children's entertainer, he would throw all sorts of paint on the canvas, often repeating the phrase 'Can you tell what it is yet?' The overall picture was never clear, but finally, at the end of the show, he would refine the canvas with some final adjustments, and sweeping strokes of a pallet knife or brush. Finally, as the camera panned back, the picture was revealed as a very clever picture that had been there all the time but just needed tweaking to reveal the finished article. The point is, for most of the show, he was just applying the building blocks onto the canvas.
This is what I am encouraging. Get your ideas down. Don't worry if the chapter fits into what you have written already, you can sort it out later. I guarantee that when your story is complete, the feeling you get will rekindle your enthusiasm and you will set about the more boring bits with a refreshed vigour.
The fun is in the writing. Do yourself a favour and get your story down while it is fresh in your mind.
One last bit of advice before we move on to the process, is to make notes as you get ideas. I dread to think how many times I have had an idea and thought, I'll make a note of that later, only to forget what it was.
One night I woke up after having a particularly vivid dream. I was very thirsty so went downstairs for a drink and there was a notebook and pen on the dining table. I wrote down a few bullet points about the dream and went back to bed. The following morning, my wife asked me what the words meant and it all came flooding back. Those notes have now become my third novel, Mortuus Virgo. The point is, If I hadn't wrote the notes down that night, I may never have remembered them and Motuus Virgo may never have been written.
Now, go and write something..