You need to put some thought into it and ask yourself questions about it. Where's it going to take place? What's the time period? Who is the main character/s? What is the conflict that will arise if any? and so on.. I say this because it is important when writing a novel, to have these things covered. Also, if you wish to write a great book, take your time with it. Don't rush, just for the sake of getting it out there. If you do this you might have that feeling of: "Well my book is good, but if I had put more time and thought into it would it have been better?" You must consider this things because they are important factors when writing a novel.