Format it as MS-Dos (FAT) using the program Disk Utility.
thats just gay tbh, FAT32 is useless these days, it can only handle files below 4GB..
if you wanna use files larger then 4GB you will have to install software on either machine.
if you want to use NTFS - the faildows format - you can install paragon NTFS 8.0 to enable write access from the mac (it can already read NTFS)
if you want to use the mac format HFS+ you can install macdrive on the faildows machine to enable it to read and write to HFS+.
both these formats support files up to 16 EB (and yes i men EB), the hard drive capasity is however usually limited to 2TB by the disk hardware, but then again most people don't have 2TB disks yet.