First Marathon In 4 Months. How Do I Train?

So I've decided I want to do a marathon. There is one being held near me in about 4 months time and I want to start training now. What should I do? I would love to do it in sub-3:00 but I don't know if that's realistic or not. I am 17 and moderately fit at the moment. (5k in less than 30 minutes) So a bit of advice?