Firstly, a warm welcome to Mac-Forums.
Time Machine requires an "empty" External Hard Drive (EHD) which is formatted appropriately.
This is logical because Time Machine requires a specific format and in doing this there should not be any other data on it.
So, from your viewpoint, you need either a new EHD or one which you already have, in which case you have to erase all data already on it (if there was any on it already) and then allow Apple to format the drive.
This is where we need to know which Operating System (OS) you have because the formatting is, to an extent, dependent on the OS.
Assuming that you have Monterey or Ventura, the format is APFS (Case sensitive) and your Mac will do this for you.
Perhaps you could help us by giving us your Mac, model, year and Operating System.
It's really very easy to do
Ian