Where in Tokyo is your origin? Access Tokkyu is indeed a cheap option (1/2 the price of Skyliner), but keep in mind that the train itself is a regular commuter type train with mostly long seats. Plus it can be confusing unless you study the timetable and look at the signs well. Depending on where in Tokyo you're starting your trip, and depending on the time of the day (eg, rush hours), it may be worth it to just take one of the more expensive options (eg, Skyliner, Narita Express, Limo Bus).

Beep88 gives good explanation, and you want to get on the right train. Any train heading in the direction towards Oshiage with the word "Access" or "Airport" that has the picture of a little airplane on the train's signage would be a good bet. I've been confused before by the different nomenclature of the trains they use on this route... some trains are called Airport Kaitoku, others are called Access Tokkyu. I think I've gotten on "Airport Kaitoku" at one station and the same train changed its name to "Access Tokkyu" by the time I got off at NRT. To further complicate matters, I think Toei Asakusa Line has Kaitoku service which is different from Airport Kaitoku. More straightforward to take this thing (Access Tokkyu) from NRT into Tokyo, not to mention you're pretty much guaranteed to be able to sit starting at NRT-T1. But you're probably better off doing the 1500y NEx option going NRT to Tokyo.