Is it a Hard Drive or an SSD?
Was Filevault enabled on this drive?
This is very important information especially if the drive is a Hard Drive, since a simple erase of an unencrypted Hard Drive will still retain your data that is accessible using data recovery software.
What is the exact model of this Mac? And does it still boot? You can get the exact model by clicking the Apple menu and selecting "About This Mac", or if it cannot boot, then you can get the exact model by entering the system serial number on the check coverage page here (don't post the serial number on this forum as it is considered personal information):
View Coverage - AppleCare & Warranty
There are several options for destroying the data on an unencrypted Hard Drive, but which option is easiest depends on the exact model of the Mac and whether it still functions. Of course any option requires the Hard Drive to be healthy.....a failing Hard Drive may not be able to complete a secure erase or encryption process.
If your Mac is still functional and uses a Hard Drive with Filevault enabled, or an SSD, then a simple erase with Disk Utility is sufficient to destroy your data. In fact, here is an Apple article with instructions for transferring ownership of a Mac to someone else (even a recycling center):
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
Otherwise you need to provide the requested details to find out your options.