Hi, it will depend on what Passport model did you have and under what OS would you like to use it. Some Passport models come formatted for Mac (HFS+) other for windows (NTFS). Check the links below and see what Passport model you have.
Passport for Windows
http://www.wdc.com/en/products/external/portable/
Passport for Mac
http://www.wdc.com/en/products/external/portableformac/
If you need to format your Passport, follow the steps from the link below.
How to partition and format a WD drive on Windows (8, 7, Vista, XP) and Mac OSX
