The easiest way to go to Thassos is with the aircraft. You will land at the airport Megas Alexandros in Chrisoupoli Kavala, drives a taxi for a few minutes to the ferry terminal of Keramothi. In the season you can take a ferry every half hour to the island to Limenas. The journey takes about half an hour.
From the port of Kavala there are also ferries to Prinos on thassos. The port of Kavala is very far from the airport, and the sea journey is a little bit longer, and therefore more expensive than from Keramothi. Whose holiday destination in Prinos, can consider to travel by ferry directly here, or on the island by bus or taxi from Limenas to tour from.
Timetables and prices for ferries you will find for the Aneth ferries under http://www.anethferries.gr/index.php/en/dromologia-en . The Thassos Ferries page can be found here http://www.thassosferries.gr/english.html . Click here for the schedule and prices below the second icon (Iterneraries-Prices).
The airport of Kavala for a long time will not be served from the north of Germany, usually, the only way via Thessaloniki. Recently, however, some airlines offer flights with a stopover in Dusseldorf and Nuremberg, which then go on to Kavala. For example, in 2014 the German Wings.
Who still flies to Thessaloniki, must cover the 200 km to the ferry terminal. The cheapest option is to travel by bus. In the season it goes from the airport a bus (78N), which runs every half hour to the bus station to Thessaloniki. From here, run every 60 minutes buses to Kavala. The journey takes about 2 to 2.5 hours. The timetable can be found here http://ktelmacedonia.gr/en/routes/tid=17#routes Alternatively, a taxi can be seen, but this is considerably more expensive. Please clarify the travel expenses with the taxi driver before you start.
Another possibility is to rent a car. At the airport of Thessaloniki all large companies are represented. It should be noted that most insurance cover is not usually on the ferries. In most cases, an agreement by the rental companie for the use of ferries must be given. Also are damage to tires and undercarriage will not be covered and the car may only be used on roads. A special focus shall be put on the sums insured for liability claims. These are much too low to cover usually a greater harm. Here the so-called Mallorca-Police helps. It is an insurance policy that increases the liability coverage for a rented vehicle. The Mallorca-Police is included in many cases in the German car insurance. So let’s call the insurer of your car and ask (or read).
In any case, before hiring a car well read the fine print. If in doubt, always ask exactly the car rental.