Well if you want to use it on a mac, you need to either format it to "Mac OS Extended" or "FAT 32." Windows can only write, read, and format for NTFS and FAT32.
What you do on a windows is plug it in, find it on "my computer" select the drive and right click, select 'format' and go to work.
On a mac, go to the applications folder, then the utilities folder within the applications folder, and find "Hard disk ultility." Then you select your drive on the left and "erase" the drive, after selecting the proper format.