Sifnos does not have an airport; the nearest airports with international flights are in Mykonos and Santorini islands and of course in Athens. Search for direct or connecting flights to Athens from all over the world through the website AthensFlights.gr.
When you have reached one of the above airports in Greece you will need to take a scheduled ferry to Sifnos.
From the International Airport of Athens (Eleftherios Venizelos) take the public express bus X96 that will drop you off at the port of Piraeus, which is actually the major port of Greece. From Piraeus port you can buy your ferry ticket to Sifnos Island from any of the travel agencies that are located all around the port. Have in mind that during the summer period (mostly during July and August), the port of Piraeus is an extremely busy place with thousands of people trying to visit the beautiful Greek islands. Therefore it is important to book a ticket in advance and in case you decide not to you must be prepared of the possibility that there might not be any available tickets. For online ferry ticket bookings please visit the website www.aktoploika.gr/en/
The duration of the trip from Piraeus to Sifnos is about 3 hours and 10 min. by speed ferries whereas the scheduled ferry boats make the journey in 5 hours and 30 min. The price for a one way ticket is approximately 47€ for the speedboats and 28€ for scheduled ferries (approx. prices 2010).
There are at least two weekly scheduled ferries from Santorini to Sifnos Island. The duration of the trip is more or less 4 to 5 hours, depending on the ferry company that operates the service. The single ticket costs 14€ (approx. price 2010).
Please note when you are searching for online ferry bookings you might not find the port of Santorini under this name but under the name Thira.
Unfortunately Mykonos does not have a direct ferry to Sifnos. Therefore if your flight lands in Mykonos you will have to take a ferry that reaches the islands of Santorini, Milos, Serifos or Folegandros and from those ports take another ferry to Sifnos.