you will arrive in Napoli Centrale
. Then take the bus to the harbor called Beverello
(2km - about 1.50 EUR).
There take a normal ship (not the super-fast-ones). It is about 5 EUR one-way and needs 80 minutes. You can catch a ship every 60 minutes, in the main times and in summer every 30 minutes.
Public buses at the island of Capri are not included in your Interrail pass - there are no trains on Capri!
To visit the Blue Grotto
, you can take a ship from Molo Beverello
where your ferry from Napoli will arrive.
Don't do it on weekends: too much people !!!