にする

1. 1To decide on or choose a noun from among options, often when ordering or making plans.Ex. 1
2. 2To propose or resolve on a choice with the volitional form にしよう, as in 'let's go with' something.Ex. 2
3. 3To deliberately make something become a certain state or thing (AをBにする).

Neutral and very common in everyday speech, especially when ordering food or drinks, choosing items, or settling plans. The ます form sounds polite, and the volitional しよう proposes a choice. Unlike なる, which presents a change as a resulting state, にする presents a choice or change as someone's deliberate act. For decisions about actions, ことにする attaches to verbs instead.

• ことにするN4「にする」attaches to nouns and な-adjectives; to decide to do or not do an action, use 「ことにする」 (dictionary form or ない-form +). 「ことにする」 also has a 'just treat it as...' sense that 「にする」 lacks, so the two are not interchangeable.
• なるN5「なる」 is close only to 「にする」's second sense of turning an object into a state: it is intransitive and describes the subject itself becoming something (whether a natural change or the result of effort), whereas 「にする」 has the subject actively change an object. 「なる」 carries no nuance of choice or decision, so the two are not interchangeable.
• くするN5「くする」 attaches to い-adjectives (drop the い and add く, e.g., 「安くする」「よくする」) and covers only 「にする」's 'make ... into ...' sense; the two divide the work by word class and are not interchangeable, and 「くする」 has no nuance of choice or decision.
• ことになるN4「にする」 attaches to nouns and the like and means the subject actively chooses or changes something, while 「ことになる」 attaches to verbs and describes arrangements formed by others or by circumstances (it can also tactfully report one's own decision) or a calculated result ('it comes to'). The two are not interchangeable.
• つもりN5「にする」 attaches to nouns to express choosing, as in 「お茶にします」; 「つもり」 mostly attaches to verbs and expresses the speaker's own intention. 「お茶のつもり」 means 'treating it as tea,' so it cannot replace 「お茶にする」.

にする presents a choice or change with nouns and な形容詞 as someone's deliberate act, while なる focuses on the resulting change itself, and くする is the equivalent of にする for い形容詞. ことにする and ことになる attach to verbs, contrasting a decision presented as the subject's own with an outcome presented as decided or arranged without foregrounding who decided; つもり expresses an intention one holds rather than the act of choosing.