First of all you must go to Naha, in Okinawa Prefecture.
There are several ferries departing from Kyūshū island, and travelling by plane is also confortable.
Naha Airport is connected to Japan’s major airports (Tokyo, Osaka, Fukuoka, Nagoya, Sendai, Sapporo etc…) with direct flights.
Once you got there, you have to go to Naha’s port Tomari futō quay. You can go there by bus from the airport, getting off at “Tomari Takahashi” and then walking for a minute.
Otherwise, you can take the Yu Rail (Naha’s panoramic monorail): you have to get off at Miebashi Station and then walk for a minute.
“Ferry Tokashiki” and “Marine Liner Tokashiki” ferries ride from Naha’s port to Tokashiki’s port.
The first one runs (retourn journey) once in a day, and the ride takes about 70 minutes. The second one is a high-speed ferry, it runs twice in a day (or three times during the Summer) and takes you to Tokashiki-jima in just 35 minutes. (details here: http://tinyurl.com/k72bvjz)
At Tokashiki’s port you can take a shuttle bus, usually belonging to a guest house, so make arrangements while booking your stay. There’s also a public transportation service, the “Tokashiki Kankō Bus”. (details here: http://tinyurl.com/l5knz5g)