‘-기로 하다’ is used to express a promise or decision to yourself or to another. It is added directly to the verb stem, regardless of whether the verb stem ends in a vowel or a consonant. This form indicates a decision or a plan and is usually expressed in the past tense(-기로 했다).
● 저는 담배를 끊기로 했어요. (I decided to quit smoking.)
● 우리는 이번 주말에 여행을 가기로 했어요. (We decided to go on a trip on this weekend.)