Do you simply not know how to format a hard drive? Please provide more information, I can't help you unless I know more. A good place to start would be with why it needs formatted and what Operating System you use.
Also, hard disk drives come pre-formatted and unless you need to wipe the hard drive it is likely already formatted in NTFS or FAT32.
In any case, this link will probably answer your question, just remember to format with exFAT. It is a very bad idea to use FAT32 as it has a 4GB file size limitation
. Anyway, the link: http://www.mactip.net/guide-how-to-format-an-external-drive-to-work-seamlessly-with-macs-and-pcs-without-third-party-software/