Before upgrading from 1.7 to 2.5, please make a backup and consult with Joomla’s documentation:
http://docs.joomla.org/Upgrading_from_Joomla_1.7_to_Joomla_2.5
Then check out the Joomla 3 FAQ:
http://community.joomla.org/blogs/community/1683-joomla-30-faq.html