Need Help with Screen Sharing with my Father

My father and I both have new iMacs (only about a year old). My father is older and he sometimes needs help doing thngs on his computer. I remember that there was a way that I could (over the internet) gain control of his computer and remotely operate it so I could talk with him and show him at the same time how to do things. I don't know if this can still be done but am more and more needing to do this.

How do I do that with our current two iMacs?