Resetting a Mac is simple. Resetting a Mac in the wrong order, however, can be very costly, and the cost will never be recovered. Therefore, prior to erasing all data from the computer: remove the files (and transfer them to another device), and remove the Mac from your Apple account. These two actions, in this order, and then the wiping process becomes the simple part it should be.
Taking the files off means creating a copy that you have actually looked at. A Time Machine drive is only a backup if you've actually restored from it. Plug it into your computer, open it up and look for yourself to make sure the files you need are there. Sign out of everything (Apple ID, iCloud, iMessage), and disable "Find My", as this is an easy thing for folks to miss and it will leave the person buying your computer with a machine locked to your account. If it is going to be sold or handed on, also un-pair all of your Bluetooth devices, simply so they do not confuse the next user.
Apple Silicon Macs and Intel Macs that have a T2 chip use the most simple method. To do this you will open System Settings, then select "General". Next select "Transfer or Reset". The option you are looking for is called "Erase All Content and Settings", which is similar to how iPhones work with their "erase all content and settings" function. This feature will sign you out of any Apple service you may be using, erase all of your data as well as your computer's settings, and put your computer back to the initial set up process, without you needing to reinstall macOS by hand.
What you need to understand is how it works behind the scenes, because it explains why there is no turning back. Encryption is used to secure data within those machines at all times with hardware. An "erase" operation does not methodically overwrite each file, but rather eliminates the encryption keys. Once the encryption keys are eliminated, what remains is true (mathematical) noise, and no lab on earth, including our own, can recover data from noise. Anyone who tells you otherwise is selling something.
If your Mac has no T2 chip, there's no single switch, which means switching back to the old way of doing things will be required. Hold Command + R as your machine starts up; this should allow you to enter macOS Recovery. Open Disk Utility. Select "View" from the top menu bar, and check "Show All Devices". Now instead of viewing just the volumes on your internal hard drive or SSD, you'll see the actual physical disk. Highlight the top level of your internal drive, click the "Erase" button, and select APFS if you have a reasonably modern machine; otherwise select Mac OS Extended (Journaled) for a genuinely old one. Quit Disk Utility. Then choose "Reinstall macOS".
Two important notes. The reinstall needs to access your Wi-Fi and takes some time to run, so you may want to do this at a location where you have good internet and with the charger plugged in. On older Intel Macs that were not protected by FileVault, an erase does not equal a shred. When you perform an "erase" the files are simply removed from the directory structure of the drive, but they are still recoverable. If this is a used system for sale which contained sensitive information, enable FileVault protection prior to performing an "erase". Otherwise consider destroying the physical hard drive.
Be clear about what you are choosing. The "erase" function is a total (or "final") delete on all newer computers running Apple Silicon processors or T2 chips. This is ideal for those who wish to sell their computer. However, if you discover later that some of your photographs weren't ever actually copied to another device, this can cause major problems. Older Intel-based Mac computers that don't use FileVault get a superficial deletion. A competent recovery will often bring plenty back, which is acceptable should you make a mistake, but completely unacceptable when the machine has just been sold through an online auction.
Storage soldered directly to a logic board, as in modern Macs, is something we will not take on. If the machine is running at all, save your data yourself before you erase anything; that is the whole window you have. The key removal process leaves no room for error: once the keys go, regardless of how many bench experts or repair shops you visit, they won't be able to restore them either. We'd rather let you know now as opposed to taking your money only to tell you later.
Sometimes you have to perform a reset because there is already some kind of problem: the computer won't boot; the hard drive isn't being detected; there's a "?" folder at startup. This is not a reset issue, and using an erase would actually delete the only copy of your information in order to try to correct a fault which can't be fixed by erasing.
In that event, please just stop. If the Mac has a drive that unclips or unscrews (most iMacs, and MacBooks up to a certain vintage), that drive can be read on our bench without needing to know if the Mac will ever start again. We can provide a price quotation over the telephone, or you can use our complimentary online diagnostic. Once we have provided a quote, you can send us the drive by post, properly packaged and sent tracked and fully insured, or bring it to the Cambridge desk. There is no collection service available to collect your device. One parcel, one phone call. The diagnostic is free, finished within 2 working days of arrival, and gives a fixed written figure before anyone commits you to anything.
Files out first, wipe afterwards: there is no later stage at which the order stops mattering. Both ways to erase a Mac are final. Once the keys inside an encrypted Mac's soldered storage are wiped clean, no lab is ever going to retrieve anything from them.
Free diagnostic inside 2 working days of arrival and a fixed quote; on logical faults you only pay if we recover your data. Start online or ring the freephone now.