All words of more than one syllable have an accent. The accent goes on one of the last 3 syllables. Which one? That you will learn by seeing the word just as you learn the spelling of the word.
The accents are used to show which syllable is stressed for example in Greek we say: "μαμά">"maMA" why is it on the last syllable...that is a matter for each language and where it comes from we don't know. Just learn the accent when you learn the word.
Hi, please check the sticky posts in the course's discussion board: https://www.duolingo.com/topic/936. Also, don't forget to check the Tips and notes section under the lessons (the section is only available on the full desktop website, not the app). There you will find all the information you need to get you started and much more! Welcome to the Greek course!