Start by opening the Calendar site.
In the top left corner, click the button that says "create".
A drop down menu will come up. Click "event". A smaller screen while then show up in the middle of the screen where you can add a title to an event, pick the date and time of the event, or even add guests, locations, and descriptions for the event.
Once you have filled out all of the information for the event, click "Save".
This will add an event to your calendar.