Begin by writing an outline. An outline will plan out the whole novel including the plot and any other interesting stories you want to include in your novel. Make a separate page for your characters.
Go back through your outline and put in more specific details about each chapter and character. Jot down notes in the margins of ideas you have that are not fully developed.
Write your first chapter using your outline. You can either write quickly or slowly and take your time. Make sure the first chapter hooks the reader in to the story. Don't tell details, show them.
Work through the rest of your chapters in the same fashion until the book is done. Don't worry about making spelling or grammar mistakes, those will be checked later. Keep working through it and don't stop until you are done.
Start at the beginning of your book and go through each chapter carefully to fix those grammar and spelling mistakes. Also read through each chapter to see if it makes sense and fix those sections that need it.
Find friends or family that are willing to look at your novel and give you constructive criticism on it. Take their advice and recommendations and incorporate them into your story.