To know the file structure for your import, I recommend you to export a customer from Odoo first. Create a customer in Odoo manually and fill the fields you need. Then export that customer in Import-compatible format by selecting the fields you would like to import back. You now have the column headings from Odoo. Use the exported column headings in your csv file with customers to be imported.

For more information about importing (leads for that matter, but the basic principles can be applied for customers too), please refer to the topic https://www.odoo.com/forum/Help-1/question/How-can-I-import-lead-databases-53964