How Do I Report a Miracle?

I infer you asked for the intercession of a particular candidate for sainthood regarding your reported miracle. If so, through an internet search you should be able to find the postulator and related association that is advancing the person’s cause for canonization. Perhaps check with the local Catholic diocesan chancery if that effort fails.

If the reported miracle you witnessed involved the intercession of a canonized saint, you might want to report that to your local diocesan newspaper, as someone there may be interested in the story. And you could possibly let the local diocesan bishop know too. But the Church normally investigates closely only miracles that regard the canonization causes of holy Church members.