You use this when you choose something. It shows you made a decision. It means 'to decide on' or 'to choose'.
Noun + に + するPrimary
な-Adjective + に + する
You use this when you choose something. It shows you made a decision. It means 'to decide on' or 'to choose'.
飲み物は何にしますか？ — 私はコーヒーにします。
What will you have to drink? — I'll have coffee.
来月の旅行の行き先は、沖縄にしました。
We decided on Okinawa as the destination for next month's trip.
部屋をもう少しきれいにしなさい。
Make your room a little cleaner.
明日の会議の開始時間は、午前10時にしない？
Why don't we make the start time for tomorrow's meeting 10 AM?
予算を半分にすれば、そのプロジェクトは実行可能だ。
If we make the budget half, that project will be feasible.
Imagine you are picking something. This grammar shows you are making a choice right now. Like when you order food, you say "I'll have the coffee." This tells people what you decided.
This is different from "〜になる" (ni naru). "〜にする" shows you choose something. Or you make something change. "〜になる" shows something changes by itself. No one made it change.
Watch out: You use '〜に する' with nouns and な-adjectives. Do not use it with い-adjectives. If you want to make something more 'い-adjective', change the 'い' to 'く'. Then add 'する'. For example, say '大きくする' (make it bigger). Do not say '*大きいにする'.
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