Folegandros is a main stop on the ferry line Santorini - Milos - Piraeus (usually 4 weekly stopovers), but it also has frequent ferries hydrofoils and Catamaran to Milos, Ios, the near Sikinos and other islands, though the exact list of interconnected islands and timetable changes from year to year.
You can find more about the Ferries schedule at www.gtp.gr
As Folegandros has no airport, the only way to reach the island is by boat. If you are traveling from abroad first fly to Santorini. The boats from here are less crowded and less expensive than the tourist boats.