You may not need to erase it all if you don't want to. What is the format of the drive right now? If it is NTFS, it will be read-only on the Mac. You can install the Paragon NTFS driver to gain read-write functionality. If it is FAT, you should be able to use it as is (within the limitations of the FAT file system).
Otherwise, to format as a native Mac disk, you would generally choose Mac OS Extended (not case-sensitive or encrypted).