You agenda uses Day entries to organize events and other scheduled items. To create a new entry, go to the Day tab then select the date you want to create the entry on in the top right corner of the left sidebar:

 

 

Now select Edit menu > New Entry > New Day Entry.

 

The new entry is created near the current time:

 

 

Double click on the entry to open the Details dialog for it, where you can change the scheduled time, set alarms, and categorize the entry:

 

 

See also

Edit details of an item

Use categories to organize your entries