As the island of Korcula is not connected with bridge to the mainland, you will have to take the ferry, provided by two competing companies: www.jadrolinija.hr .
There are three major access routes:
1) from Split
a) for cars and people
b) only for persons
2) via Peljesac
a) to Orebic
b) from Ploce to Trpanj and Orebic