how do i secure my photos on my mac?

If you are referring to the shutdown of My Photo Stream as described in Information about the My Photo Stream shutdown - Apple Support , you are not affected if you don't have a My Photo Stream album on any of your devices in the location shown in that support article. If you do have that album with photos in it, save the photos using the instructions in the support article.

Loss of My Photo Stream means we'll need to use AirDrop or cable transfer to get photos f rom our iPhone's Photos library to our Mac's Photos library. Only a small inconvenience. It will not affect your library or its contents in any way.