To create a new partner (a company, customer, supplier, ...) or to display the list of existing customers, use the menu Sales --> Customers. This menu does not only allow you to create a new partner, but also to search for partners.

Click the Create button to add a new partner. Tick the Is a Company? checkbox to indicate that the new customer is a company and not a private person. You will notice that a new tab page Contacts is added in front of Internal Notes.

You should at least enter the company's Name in the partner form. Some fields are text fields, other fields may be linked to existing data that have been entered elsewhere, such as the country. 

On creation of a new customer, the Customer checkbox is automatically ticked. Why is it important for you to correctly set the Customer and Supplier checkboxes in the partner form? These checkboxes are designed to enable OpenERP to quickly select the partners who should be displayed in some drop-down boxes. An example: when you select a partner in a Sales Quotation, OpenERP will only allow you to select from the list of Customers. And that is precisely what the Customer checkbox is used for.

To add contacts for this company, make sure to be in Edit mode. Click the Create button on the Contacts tab and type the Contact's name in the blue Name field. You can use the company address or add a specific contact address.