how do I determine which are the largest files on my hard drive so I might search for files to delete to make space?

DaisyDisk, available on the Mac App Store, gets good reviews and is used by many here. This might be the quickest means of finding all the largest files.

Just sort your documents/downloads by size and review. Usually videos and photos take up the most space. The videos/photos can be off loaded to an external drive if you need more room. Other idea is to use an external drive (larger) cloned and use as the boot drive for all.

Malware warning keeps popping up every few seconds. I have deleted the file warned about and emptied the trash, but keep getting the pop up warning.