ことにするDecide to do or not do
ことにする presents an action—or a choice not to act—as a decision made by the subject, often the speaker. It is especially natural when announcing a choice that has just been made or reporting one made earlier.
The chooser and the time of the decision
The construction puts attention on the subject's choice. Use the nonpast ending when making or announcing the decision now. Use the past ending when the decision has already been reached—even if the planned action itself will happen later.
Affirmative and negative choices
The action before こと is the content of the decision.
Personal choice or settled arrangement?
ことにする foregrounds someone's decision. By contrast, ことになる presents an arrangement or result as established without emphasizing the chooser. English often translates both with “decided,” so pay attention to who controls the outcome.
One decision, not necessarily a habit
This form can report one specific choice. If you mean a continuing personal rule or repeated policy, use the related ongoing form instead. A single decision may later lead to a habit, but this construction by itself does not emphasize repetition.
More examples
いろいろ考えて、今回は応募することにしました。
After thinking it over, I decided to apply this time.
Analyzeこの件については、今夜は返事をしないことにした。
I decided not to respond about this matter tonight.
Analyze大学を卒業したら、しばらく海外で暮らすことにします。
I've decided to live abroad for a while after graduating from university.
Analyze予算を確認して、古いパソコンは買い替えないことにしました。
After checking the budget, I decided not to replace the old computer.
Analyze雨が強いので、今日は車で帰ることにする。
Since the rain is heavy, I've decided to drive home today.
Analyze店を何軒か見てから、この机を買うことに決めました。
After looking at several stores, I decided to buy this desk.
Analyzeことに決める is a closely related expression that states the act of deciding more explicitly.