In readonly field first we have to go in .py file (python file)
After that we have to decide where we want readonly field and then give attribute readonly to that field.
See below python code in py file for reference : -

 class spartner(osv.osv):
    _inherit='sale.order'
    _columns={
              'invoice_address':fields.many2one('info1' ,'invoice_address'),
              'Delivery Address':fields.selection([('simple','simple'),('Extensiva','Extensiva')],'Delivery Address'),
             'acd_name':fields.char("Name"),
              'acd_address':fields.char("Address"),
              'acd_city':fields.char("City"),
              'adress':fields.char("Address"),
              'state': fields.selection([
                                         ('draft', 'Draft Quotation'),
                                         ('apr', 'Approval'),
                                         ('Quotation', 'Quotation Sent '),
                                         ('sent', 'Quotation Sent'),
                                         ('cancel', 'Cancelled'),
                                         ('waiting_date', 'Waiting Schedule'),
                                         ('progress', 'Sales Order'),
                                         ('manual', 'Sale to Invoice'),
                                         ('invoice_except', 'Invoice Exception'),
                                         ('done', 'Done'),
                                         ], 'Status', readonly=True,