Kefalonia is located at the west coast of Greece and belongs to the Ionic Isalnds, together with Zakynthos, Lefkada, Corfu and Ithaka.
Easiest way to go to Kefalonia is by air. From april to october easyJet offers up to three direct flights per week from London Gatwick and other airports also do direct flights to Kefalonia. Who wants to be more flexible can take a flight to Athens and another one from there to the island.
If you are not in a hurry and want to see the greek landscape between Athens and Patras/Kastro Kilini you can take the bus from Athens. These drives to Patras or Kastro Kilini and there onto the Ferry to Kefalonia. The destination port from Patras is Sami and from Kastro Kilini is Poros. You should pay attention to the fact that these need much more time to travel.
If you want travel with your own car it is suggested to take the route Venice–Patras or Ancona–Patras. Furthermore there are ferry connections from Bari and Brindisi (Italy) to Patras. (Greekferries.gr)
Do you have questions to the directions? Please ask me.