In a similar way to how にする means to ‘decide on’ something, ことにする means to decide on a situation, or course of action. This grammar point is a combination of 事（こと） ‘thing’ in its role as a nominalizer (a word that creates a noun-phrase), and にする ‘to make it (A)’. Literally this grammar structure can be translated as ‘to do (A)’, or ‘to make it (A)’.
ことにする may be used with verb-phrases that are either in the affirmative, or negative (using ない) forms. As こと itself is a 名詞（めいし） (noun), the verb (or auxiliary verb) that comes before it will need to be in its 連体形（れんたいけい） (attributive form).

• 今日（きょう）はワクチンを打（う）ったから、明日（あした）は仕事（しごと）を休（やす）むことにする。
  Today I got vaccinated, so I will go with taking the day off tomorrow.
• タマキさんと箱根（はこね）に行（い）くことにする。
  I have decided to go to Hakone with Tamaki-san.
• 今日（きょう）は雨（あめ）が降（ふ）っているから、明日（あした）行（い）くことにしない？
  It is raining today, don't you want to make it that we go tomorrow?
• 勝手（かって）に僕（ぼく）が行（い）くという事（こと）にしないでください。
  Please don't decide that I will go without asking me. (Please don't assume that I want to go)

にする is a strong expression, which tends to make the speaker sound like they are fairly resolute in their decision.