If you plan to arrive to Croatia by plane, book a flight to Croatia’s capital, Zagreb. Zagreb International Airport, also called Pleso, is located 17 km from the centre of the city. A regular bus service connects it to Zagreb Central Bus Station. The ride takes 20-25 minutes, and the ticket costs around 4 euros. (Please beare in mind that euros are not accepted in Croatia, as the official currency is the kuna.) The bus schedule can be found here.
From Zagreb Central Bus Station, buses leave for Sisak every hour. The distance between the two cities is 60 km, and the ride takes about 70 minutes. A single ticket costs around 5 euros. An additional baggage fee may apply. Visit the Zagreb Central Bus Station webpage to view departure and arrival times: http://voznired.akz.hr/voznired.aspx?lang=en (Enter SISAK as your destination.)
Another option is to take a tram from Zagreb Central Bus Station to Zagreb Central Train Station. There is a direct train connection from Zagreb to Sisak that runs daily. A single ticket costs 5 euros, and the ride takes about an hour. Visit the Croatian Railways’ webpage to view departure and arrival times: http://www.hzpp.hr/en/timetable?m=73&mp=13 (Enter ZAGREB GL. KOL. as your departure station, and SISAK as your arrival station.)