So you have decided to write a book, but don't know what to write about. This article is on writing fantasy genre books but can be used for other books as well. The best books are written by people who enjoy the genre.
If you love fantasy genre books like the Lord of the Rings, you should write a fantasy book. Start by thinking of a story. Write down notes while you think, also think of a problems that can be resolved by the main hero. Your main story will revolve around your hero or group of heroes, but you will still need one main hero.
When writing your book you should make every chapter interesting, it doesn't matter if it is relaxing interesting or exciting interesting, but should be interesting, otherwise people will stop reading the book and call it boring. The first chapter should be very fast and adventurous because that is the chapter that will grab the reader's attention. After the first chapter you can slow down the story and take the time to lay out your world for the ready to explore.
The ultimate goal to your book, killing the great evil, or destroying the one ring should be eventually realized about one third or mid way in your book. But before you realize your ultimate goal you should have some side adventures for your characters on the way to the main goal. This keeps the story from being too one purpose. When writing you should avoid grammar checking or editing. That stuff can be done after your book is completed.
After your book is complete you should first read the entire book to make sure everything makes sense. After that you should revise to make your book better. You should revise at least 3-4 times to make sure you are making the best book you can. Have others read your book too, it can never help to have multiple opinions. After you have finished revising, you will need to edit and grammar check your entire book, or have an editor do this. Make sure you enjoy writing your book, because if you aren't neither will the reader.