The first Narita Express leaves Tokyo Station at 6:18 am and arrives at NRT at 7:18 am. For a 9:00 am flight in C/J, that ought to be early enough to make it. There is another at 6:30 am, arriving at 7:30 and again at 7:00 am, arriving at 7:56 am, which might be cutting it pretty short.

You can also catch those two earliest N'Ex's at Shinagawa (6:05 am and 6:17 am), Shibuya (6:01 or 6:13 am) or Shinjuku (5:53 and 6:05 am.) They actually start earlier at minor stations but might be tough for a first-timer to find.