This is a good guide that explains how to securely delete data from a Mac, but it also references a PC. These instructions might be beyond the capabilities for the average computer user, but it’s still good to know what is involved in securely deleting data.
Remember that when you get rid of an old computer, you have no idea what information is left behind about you. Data theft is a real problem because you can be targeted in such a way as to never really know how to combat it if you do not what information is released into the world.
The best bet is to hirer me, I can securely delete sensitive data before you recycle or give away your legacy device.