Hello Joom2.5 and thanks for your question.
If you have already created a sitemap of your Joomla 2.5 site, then you should have a (sitemap.xml) or similar file now. You can upload that file to your website via FTP or the cPanel File Manager and then typically you'd want to follow Google's instructions for either having them crawl through your Sitemap via Google Webmaster Tools, or by placing a line for it in your (robots.txt) file.
Using FTPHow to upload a file in cPanel's File ManagerSubmitting a Sitemap to Google
I hope that is enough information to get you headed in the right direction. Please let us know if you have any follow-up questions at all.
- Jacob