There are many ways to get to Evia. There is no airport but you can fly to Eleftherios Venizelos International Airport in Athens and take a taxi and be on Evia in an hour. For those who want to avoid ferries you can drive north from Athens and get off at the Halkida exit and cross the small bridge that goes right to the center of town or the larger suspension bridge that is south of town.

There are numerous ferry options. From the port of Rafina, 20 minutes from the airport, you can take a ferry to Marmari in southern Evia. From the tiny port of Agia Marina which is just beyond Schinias Beach near Marathon you can take a small ferry to Nea Styra. Both trips are an hour more or less. From the town of Oropos just north of Athens you can take a ferry that goes back and forth every halfhour or so and takes around 20 minutes to Eretria.

For northern Evia there is a ferry from Arkitsa to Edipsos. The port is right off the National Road and the ferry takes around half an hour. There may or may not be a small ferry from Agios Konstantinos on the mainland to Agios Giorgios on Evia.

From Glyfa to Agiokambos there is a regular ferry that is popular with people coming from the north.

Seajets and Anes run highspeeds and conventional ferries between the port of Mantoudi, Evia to Skiathos, Skopelos, Alonnisos and Thessaloniki and supposedly to Agios Konstantinos though I don't know why anyone would want to do that. And there is a ferry that runs between Kymi and the island of Skyros. Some of these are seasonal so check Ferryhopper

You will probably want a car if you plan to explore Evia. Swift Car Rental will deliver your car to the airport or to your hotel in Athens and drive you out of the city. Otherwise you can use George the Famous Taxi Driver to get you to your hotel in Evia or to whatever port you decide to take the ferry from. You can also take the Ktel Bus to Rafina, Oropos, Halkida or any of the other mainland ports with ferries to Evia if you can figure out the schedules on the KTEL Evia site. (I couldn't).

There is also a train to Halkida from Athens or Thessaloniki and one from the airport that leaves every couple of hours. See the Greek Trains website

If you are rich you can take a helicopter.