get a good training plan. the runner's world website has a good one where you plug in parameters and you get a personalized plan.
once you have the plan, DON'T look forward to the race, or a month in advance, or even a week. Just look at your next run, and go out and do it. keep doing that, and eventually you'll be there. but if you keep considering the huge race at the end, you'll psyche yourself out.
since it's your first marathon, don't pay ANY attention to time. don't push yourself or set a time goal at all. your ONLY goal is to finish. after that, if you want to run another one, then you can start focusing on time.