I assumed that Time Machine was automatically backing up my computer. I didn't know you have to set it up??? It says to use a back up disc. My Mac doesn't have a disc drive. Can I use a flash drive?
iMac 21.5″, macOS 10.13
Do not back up to a flash drive. Most do not have sufficient capacity and they are not all that reliable and can fail. You should buy an external drive dedicated to backups. These days they are cost effective, compact, easy to install with a vendor provided USB cable and most do not require any external power although you may have to first reformat it. As noted, be sure the drive has sufficient storage capacity. Last, Time Machine does not automatically set itself up. Once it is set up it does its thing aotomatically, though.
You must use a separate drive connected to your Mac. It can be a disk drive or a solid state drive. Recommend it to have double the capacity of your Mac. Short answer, shop your nearest Apple store for one and ask how to set it up.
How to use Time Machine
