The quickest way is to take an express bus (either E94 or E95 to the underground (METRO) station "ETHNIKI AMYNA". (E94 stops there, E95 continues until SYNTAGMA Sq. From there take the METRO to "LARISSA STATION" (for the trains). The bus station (A KTEL at Kifissou Street) is not far away from there, but one needs a taxi or other transport.
The buses (E94 or 95) and the Metro run very often. One should buy a 1000 drachmas (~3 EUROS) ticket. This is a DAY-TICKET and allows one to take any public transport vehicles in the greater Athens area (including ONE ONLY Express bus - e.g. the E94 (mumber starting with "E")