You would have to manually move every data file of interest to the Trash/Wastebasket and then empty the Trash (via Empty Trash in the Special menu). You will have to look for personal files in various folders. Preferences files can usually be deleted as well.
With a Mac OS 9.x iMac, files emptied from the Trash will not be securely erased. That is, the files could be recovered unless overwritten. So, once all affected data files have been moved to the Trash, and the Trash has been emptied, you would have to use a third-party utility in order to overwrite (securely erase) free space.
The commercial Norton Utilities contained a utility called Wipe Info, at least for earlier Macintosh system versions. There was a free utility called Burn 2.5, too (https://www.nic.funet.fi/index/info-mac/_Disk_&_File/). Check compatibility with your exact operating system version and its file system before using any utility.