The buses from the Helsinki airport to Turku depart every hour, typically 10 min to the hour but this may vary. Please check the schedule. The bus will depart from bay 14 outside the international terminal (they will also stop at bay 3 near the domestic terminal). Directions to the bus stops are found in the map over the airport.
The travel time is approximately 2h45. You will need to change buses near Helsinki but this is a well organized and smooth process. The bus will go via the city of Salo from where it still takes an hour to Turku.
The fare is EUR 27.50/person (changes are possible). You may pay this on the bus (most often to driver, in some cases there will be an attendant). All major credit cards are accepted. A ticket can also be order electronically at the Matkahuolto site.