BACK UP ONTO A HARD DRIVE

To start a backup, you need an external hard drive. Here’s an excellent one sold by Apple: G-Technology 4TB G-DRIVE mobile USB-C Portable Hard Drive - Apple. You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support.

Also, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:

• Upgrade to iCloud+ - Apple Support
• Set up iCloud Drive - Apple Support
• Add your Desktop and Documents files to iCloud Drive - Apple Support
• Set up and use iCloud Photos - Apple Support

Jack

See the information in Back up your Mac with Time Machine - Apple Support

Time Machine is included in macOS.

Jack-19 wrote:

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Also, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine.

...

Jack

Hi Jack -- Having an additional offsite copy is an excellent recommendation and something that I do myself -- but you may want to add the caveat that if iCloud is used (for Photos or iCloud Drive files) and if the Optimize Storage option in Photos or the Optimize Storage option for iCloud Drive in System Preferences is turned on and if the Mac is low on storage (so that some or all of the data is actually optimized), there may be no local copy. In that case, Time Machine will not back it up and the one and only copy will be on iCloud. I've never had a problem with iCloud, but I don't trust it enough to have my only copy there.