You would need to know what your web site consist of and thus what should be backed up/restored. Usually sites are composed of files and databases. You must take care of them all if you want to have your site exactly the way it has been before.
There are two parts to a complete back up of your Joomla site.
- The database
- And the other is file system
Instructions on how to backup a database using PhpMyAdmin is here.
or Alternatively, there are several backup extensions for Joomla 1.5. (though u mentioned here don’t want to use a plug-in, however u may use Akeeba Backup utility)
For File Systems. besides the DB, you should also back up ur site files. The interval for this depends on how often you or your users make changes to the file system. If users can add photos and media or you make changes to the site other than simple text edits, it should be done often.
NOTE: This is best done with an FTP program and is no different than backing up a standard website. Download all the files and folders that exist in the main directory, and save it to a folder on you local computer. Be sure that the file and directory structure remains the same as it is in the live site. You could then burn a CD or save to an external hard drive if you desire.
CAUTION: Most hosts provide an FTP capability from within their control panel.
Hope this helps you!