2011 IMAC desktop

How to back up your Mac - Official Apple Support

To reply specifically to your question, you need to have a medium to back up to. Most of us purchase an external drive to back up to and then use the Time Machine program, located in your Applications folder, to schedule back ups. On the initial back up, Time Machine will back up everything. Subsequently, it will back up things that have changed. Eventually your back up drive will become filled up. Not to worry, though. As additional back ups are done, the older ones are overwritten, so you always have a choice of which back up to use if you need to. Be sure to get a back up drive that is larger than your existing hard drive. Most will connect to your computer with a USB connection. You have the choice of two kinds of drives; rotational hard drives and solid state (SSD) drives. SSDs are faster, but cost more money. You don't need speed for backing up. There are many good brands of external drives available. Most all will work fine. One final note of caution. If the drive comes with its own formatting/maintenance software, I strongly recommend NOT using it. They often cause problems. If you have further questions, please post. You are smart to do a back up. At some point your old hard drive is going to fail and without a back up, all of your data will be lost. Good luck.