〜にする
decide on, go with
Noun + に + する
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Noun + にする means "decide on" or "go with": 私はコーヒーにします, "I'll have coffee". It's the everyday way to choose from options, and you'll hear it at every restaurant table.
The past, にしました, reports a decision: 旅行は京都にしました, "we went with Kyoto for the trip".
Compare なる: 〜になる is something becoming what it is by itself, 〜にする is someone choosing it. 静かにする is "make it quiet, keep quiet", which is why 静かにしてください means "please be quiet".
To ask what someone wants, 何にしますか, "what will you have?"
Examples
私はコーヒーにします。
I'll have coffee.
飲み物は何にしますか。
What would you like to drink?
旅行は京都にしました。
We went with Kyoto for the trip.
じゃあ、この赤いのにします。
Okay, I'll go with this red one.
図書館では静かにしてください。
Please be quiet in the library.