Your question seems to be all over the place. However you, the Bakery Single Sign-On System module sounds like what you need.
Bakery provides a "single sign on" feature for Drupal based sites that are on the same second-level domain (i.e. example.com, subsite.example.com, subsite2.example.com). It could also provide support for any other website that implements the same web cookie, xmlrpc, and POST methods.
This module started at the 2009 Drupal.org redesign sprint in San Francisco as a way to manage single sign-ons across *.drupal.org infrastructure.
Read the bakery documentation for more information.
