Create event
Go to the calendar by clicking on the "Calendar" button in the left hand menu.

The window that features the ‘create event‘ form can be accessed by clicking the calendar time grid (you can also drag & drop to select longer interval).
Enter the event name, adjust the time if needed, select a resource (can be a meeting room or a company car), add your colleagues and write agenda if you need to add details about the event. Finally, click the save button at the bottom of the form.
Private event
You can create a private event if you check the "private" box under the datetime fields.
info_outline Note that if an event is private then only members of the event can see the details about it. Other people will only see "Private event - [your name]", so they will know you are busy at the specified time.
All Day event
If you know that the event you are creating will last the whole day, simply check the box under the date & time fields. Your event will then be displayed at the top of the calendar.