Under the People navigation item on the left, the customers screen allows you to add/edit/delete customers.
Initially, your customers screen will be empty. Click the Add Customer button in the top right to open the Edit/Add Customer window.
You can add general contact details for the customer, record notes, and view any associated Vehicles and active Jobs that are associated with this customer.
Hit save and you'll see the new customer appear in the Customers list. From there you can see the last time the customer was updated, and also delete or edit the record.