Getting to Narita Airport in the morning

hi

I have a flight from Narita airport to Seoul at 9:20am.

I see that the first train from Tokyo is around 6:11 and takes about 1hour. will it be enough time?

Ideally it should be OK, assuming no delay of the train. It takes about 10-15 minutes on foot from the track to the check-in counter.
However, when I catch morning flight, to be sure, I take JR Yamanote Line (usually starts service as early as 4:30 am) to Nippori and jump into a Keisei Line limited express train. The one fair from Nippori is 1,000 yen.
You may also want to explore the bus as an alternative, if you plan to go to the airport early. I think they have service starting from 5:30 am from central Tokyo. It will take you to the door of the departure terminal (check-in counter).

so the best station in Tokyo to catch the train to Narita is Nippori?

I reckon getting to the airport 2 hours before flight should be enough.

And it cost only 1000?

I think trains in Japan are more in time than anywhere else. :)

The best station to catch the train to Narita Airport depends on where you start.
http://www.japan-guide.com/e/e2027.html

If you have an early flight like that, the best is definitely to spend the last night at a hotel close to the airport.