Jisho

が is how you can build your example sentence.
If you're wondering how to conjugate 亡くなる into its past form, click on "Show inflections" next to the entry. It will show you the following possibilities:
* 亡くなった
* 亡くなりました

Note that there are also other ways to say that someone/something died or doesn't exist anymore.

Formatting in this forum makes some parts of my post invisible. I meant you need to say:

Nounがpast-form of 亡くなる