Here are a few suggestions to get you going.
1) Start with an outline. You need a map to keep you headed in the right direction. Begin with a synopsis of the book and then move on to outline the chapters.
2) Set a timeline. Most of us have full time jobs outside of writing so I recommend setting up a weekly timeline. The first draft should take you somewhere between 90 - 180 days.
3) Begin writing and don't stop to make edits, just keep writing.
4) Once you complete your first draft go back over it and compare it to your outline. Make your revisions.
5) Have several trusted friends read your work. You may want to join a writers group for input too.
6) Enter contests.
7) Find an agent or self publish.
8) Market the heck out of your masterpiece and treat yourself to a book party!
Happy writing! And, let me know what challenges you have.