Ko Chang is located close by the border of Cambodia. Hat Lek or as it is known by the Cambodians, Cham Yeam is where the borders meet. Daily mini buses will bring you from Cambodia’s Capital Phnom Penh to the Thai border. Your hotel or Guesthouse should be able to arrange this for you in Cambodia. The border closes at 9pm so ensure you arrive before then to enable your crossing.
Once you have successfully crossed the border there are minibus services to Trat that leave every 45 minutes taking an hour. They shouldn’t cost you anymore than 100 baht. You will then need to take a bus, Songthaew or taxi to Laem Ngop to catch a ferry to Koh Chang.