There are 3 methods to add to a new client:

Method 1 Customer Module

Go to the "Customer" module and click on the button "+ Add customer".

Method 2 Module Quotes

Navigate to the Quotes module and click on "Add Customer"

Method 3 (Upload in bulk)

Navigate to the Customers module and click on "Upload New Customers". If you want to learn on how to add clients in bulk check this link  
How to add bulk customer list?