You need to start by reading the implementations in the osv.fields module. In nutshell, all fields are inherited from _column. Depending on the nature of the field you are creating, you might need to implement your own "set", "get" and "serach" methods to handle the database interaciton. Here is small example to get you started. You can read more from the sourcecode of fields.py

from openerp.osv import fields, osv
class mynewfield(fields._column):

    _type = 'char'
    def __init__(self, string="unknown", size=None, **args):
        fields._column.__init__(self, string=string, size=size or None, **args)
        # self._symbol_set_char defined to keep the backward compatibility
        self._symbol_f = self._symbol_set_char = lambda x: _symbol_set_char(self, x)
        self._symbol_set = (self._symbol_c, self._symbol_f)
fields.mynewfield = mynewfield

Having saed this, I strongly recommand you to try and reusing some of the existing fields and if you need specialized display in the GUI or you need specilaized GUI editor, you can write a web Widget in JavaScript withouth creating a custom fields in the OSV. There are good tutorials how to create new Widgets and are asy to follow.