External Hard Drive how to use with iMac?

After you have copied anything you wish to save, open Disk Utility. Click on View and select Show All Devices. Select the device you want to reformat in the left sidebar. Select the main drive, not the volume below it. Click on Erase at the top of the window. Select Format: MacOS Extended (Journaled) and Scheme: GUID Partition Map. Click on the Erase button at the bottom right.

If you want to use this with both Mac and Windows select Format: ExFat.

If you used the external drive on Windows and now want to use it on MacOS, you indeed have to reformat it. Reformatting will erase the disk, so you'll loose the files that are on it now. If you think you'll need the files again, copy them to an external disk formatted as ex FAT: files on a disk formatted in ex Fat can be read on Windows and MacOs.

How to reformat a disk for Mac?

Connect the disk, and go to Disk Utility (in Applications/Utilities).

Select the disk on the left side of the window (take care and choose the right disk), click on the Erase button and choose "Mac OS Extended if you want to use it on MacOs only".

Click on erase

You are not going to use APFS. Do the other. If you have data on the external currently then you'll need to save it somewhere there use Mac's disk Utility to erase that external. Then copy your data back to it.