Time machine with a large hard drive - ideally 3 times bigger than the size of the system(s) you are wanting to backup. Best and easiest. One of the few "It just works" you still get with Apple 🙂.
Get an external USB3 hard drive with sufficient capacity. It will be more or less plug and play. Note - the drive will probably be wiped as part of the setup, so don't have any important files on it before you start.
Leave it connected to your mac all the time (or if a laptop - whenever it is home)
See:
https://support.apple.com/en-gb/HT201250
By the way:
"iCloud is not a backup"
The problem with iCloud for photos is it makes all the libraries across all the devices the same. While it does protect you from a drive failure - it doesn't protect you from accidental deletion. If you delete photos on your phone, they are deleted on your mac, and from iCloud - same if you delete on your mac - they'll be deleted on your phone. There have been rare cases of system failure also causing deletions (or just user error). There are also numerous examples of library corruption making it very difficult to access photos.
Every few days we get someone here saying "I deleted photos - how can I get them back". If it was more than 30 days ago, and there is no other backup, the answer is "you can't"