Taxes applied in your country are installed automatically for most localizations.
Default taxes set in orders and invoices come from each product's Invoicing tab. Such taxes are used when you sell to companies that are in the same country/state than you.
To change the default taxes set for any new product created go to.
If you work in a multi-companies environment, the sales and purchase taxes may have a different value according to the company you work for. You can login into two different companies and change this field for each company.