bvf

I'm off to Tokyo in November on my first trip to Japan. I'm intending to take the Narita Express to Shinjuku, but I'm confused about how to go about this. It seems you can buy tickets at the airport, but that you need to reserve in advance. Is that right? If you have to reserve in advance, does that mean for a particular train? If so, how do you know which train to book (flight delays, time taken to get out of the airport, ...) Or am I worried over nothing - can you do everything after you get to the airport?