My first question would be if you have been running regularly prior to this decision to run a marathon or not? You need to build up some sort of base. Even if it is something like 10miles/week. Depending on how much prior training you have done, will also help determine the length of training you need to do prior to your marathon. On average, I would say most training programs for marathons are 3 months. Also, another thing to consider is the marathon course and the race itself. For your first marathon, I think doing a larger one is probably more enjoyable--the San Diego Rock N Roll marathon is always really popular. You might consider doing your training with the Lukemia Society or one similar--they are generally good with first-timers. If you are still trying to decide on a race,
www.runnersworld.com has some great info on marathons and training. Good luck and enjoy yourself!