にする is a grammatical structure that joins the particle に, which marks a target or result, with the verb する (to do). It conveys the meaning of 'to decide on' or 'to choose' something. The key nuance of にする is that the speaker is making a conscious choice from a set of options, exercising direct control over the decision. The noun that precedes にする is the option being selected.

• 私(わたし)はラーメンにする。
  I think I'll go with the ramen.

In this context, while 'to decide' is a valid translation, the phrase is often more naturally rendered in English as 'I'll have...' or 'I'll go with...'. A more literal interpretation, 'I'll make it (the ramen)', helps to understand the underlying feeling of selection and finality.

The particle と may sometimes appear in place of に. While the general meaning remains 'to decide', the nuance is different. Using と softens the statement, removing the sense of a firm, personal decision. とする is often used in situations that require a bit more formality, primarily because it sounds less assertive by framing the choice as a suggestion or a temporary agreement rather than a final command.

• 今回(こんかい)はこのプランとする。
  For now, let's proceed with this plan. (Suggesting a tentative agreement)
• 今回(こんかい)はこのプランにする。
  I'm deciding on this plan. (Stating a firm personal choice)

To illustrate this difference with an English parallel, consider the contrast between the following:

に - I'll pick this one. (A direct, definite selection)

と - Let's say it's this one. (A softer, more provisional statement)

This grammar point is not just for physical objects; it can also be used when making decisions about more abstract concepts.

• 待(ま)ち合(あ)わせは駅前(えきまえ)にする？
  Should we make the meeting spot in front of the station?