Secret Information
In secrecy you must announce this information. It's not for anyone to hear, it's secret information.
みっこく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. The みつ goes to みっ.
とある会社の脱税について、税務署に密告したいと思ってるんです。
I’d like to inform the tax office about a company’s tax evasion.