How can I add a new customer or a new supplier ?

You can do this by adding a contact on the Contact page.

Either you can do that from your Dashboard by selecting Create New and then Contact.

Or you can also do it from the Contact page by selecting New Contact.

When you link an invoice or a bill to this contact, that contact will automatically be allocated to your customer or supplier list accordingly. The same contact can belong to both lists if it links with both an invoice and a bill.

Learn more about completing the information here.