How do I set up Calendar

I have used Thunderbird 's calendar but have now deleted it and want to use Calendar on my imac. I don't have any other calendars.
When I open Calendar try to add an event I get the message..."You can’t change events in the “Found in Mail” calendar."
So how do I set up an event? ie a doctor's appointment or a meeting?
I am the only user on this iMac. I am retired so do not want to share my calendar with others.
Chris

Sorted. It seems I needed to set up a new calendar. The on-line help seems to assume one would know that!