Outline your story first. It doesn't have to be detailed. Make a simple numbered step outline that addresses the major plot points in your story. Make note of events that need to happen in the novel and what needs to be set up before those events happen. Knowing your basic story before you start writing your first novel can make this step less daunting.
Have brief bios for your main characters. Character sketches help you identify with your characters and make it easier for you to write about them. Having a character sketch in front of you takes away the time you'll spend thinking about what your character should do. You'll be more inclined to begin writing more quickly and with more authority.
Don't edit as you go. Too many first-time writers spend too much time thinking about what they're writing and going back over it. Let the story tell itself. Write with an open mind and put down your scenes the way they come to you. The first draft will have to be rewritten anyway. Follow your outline as closely as possible, but don't be afraid to stray if the story seems to dictate it. You can clean up and tighten the writing on the second draft. The writing comes much easier once the initial draft is in place.
Begin at the end. This is a successful approach sometime employed by writers stuck on plot. Knowing the end of your story and writing it first can make writing the rest of the story easier.
Keep a goal in mind. A first novel is the hardest to complete. Set a realistic goal for yourself. Five pages a day is a reasonable goal. It will produce 25 manuscript pages in a five-day period. Multiply that by a month and you'll soon have 100 pages. At that rate, you're first novel will go from "beginning" to "finished" in no time.