Develop your working plan for your novel. Thirty days of writing doesn't leave much time for outlining, writing character sketches, doing research, and navigating tricky plot twists. If you've been thinking about this novel for years, chances are you have it mentally mapped out already. You'll do yourself a favor by getting some of those thoughts on paper so you have them to refer to in the heat of writing.
Determine the approximate number of words your novel will be. Novels average around 240 pages, with 250 words per page, so you're looking at around 60,000 words. You can go for less than that or for more, depending on the scope of your story. Young Adult Novels are shorter. Averages for particular genre novels vary (Western, Romance, etc.) so do a little research and determine your rough estimate.
Divide the total number of words you want to write by 30. Now you have your word-per-day requirement. If you're going for 60,000, you're looking at 2000 words per day.
Allot a particular time and place for writing every single day of your 30 days. If you are working a full-time job, it may have to be early morning or evening. You can use additional weekend time to catch up. Put the time on your calendar or planner. Let people know you are not to be disturbed.
Show up at your appointed time, sit down, and start writing. It doesn't matter if you don't feel inspired. Make yourself get words on that page. Inspiration has a funny way of hanging back to find out if you mean business or not. It will probably show up halfway through the second page, so hang in there. Keep typing and in 30 days, you'll have a novel.