Usually cases are created through the top menu "New". However, it's also possible to create cases through an account or through an existing case.
Hover on New in the top menu, and click Case in the drop-down menu.
By default, new cases are Customer cases as indicated by the darker tab at the top. Click to select the Internal tab if you wish to create a case for internal work only. Read more about the difference between customer and internal case.
If there is an existing case that's similar to the one you want to create, click to expand the Template section and use the drop-down menu to select a case. Information will be taken from the existing case to fill many of the fields, and you only have to enter a Case name, select a sales status and adjust the included data such as account name, case members, etc. to make changes.
If you're not using a template, go to the Case details section and provide as much information as possible about the new case such as the Account name, Contact, Case name, Description, etc.
In the Sales section, use the drop-down menu to select the Sales status. And provide information about the Lead, Value and Expected order date if possible. Entering the sales data is optional.
Selecting a sales status will open the Next activity section. Enter an activity related to the new case as a means to prompt your follow-up on the case in the near future.
Open an existing Account, and click New Case on the title bar of the Cases section.
Fill in the details and save.