ことにする

ことにする means "decide to": you make the choice yourself. It follows a verb in the dictionary form, or the ない-form for deciding not to. You hear it mostly as ことにした, "I've decided to".

The verb before こと stays in the dictionary form even when the decision is past: ×行ったことにした. The tense goes on する.

In the non-past, ことにする announces a decision as you make it.

ことにしている

ことにしている is a decision you made and keep to: "I make it a rule to".

ことにする or ことになる

ことにする is your own decision. ことになる says it was decided for you, by your company, by a group, by how things turned out.

Usage notes

These go beyond deciding to do something.

Treating something as so

After a た-form, ことにする means "treat it as if" – you decide to act as though something happened, or didn't.

Choosing a noun

With a noun, the pattern is にする without こと: こうひいコーヒーにする, "I'll have coffee". ×コーヒーことにする.