You just need to reformat the drive, is there anything on the drive right now? If you do, add those files to your computer (if you can) or find a friend with a pc and transfer the files, because you need to completely erase your hard drive.
Reformatting is super easy on mac, go to disk utility (use spotlight (command + space bar) to find it, easiest way).
You'll open up disk utility and you'll see your drive on the left hand side, click the top tab (will have the brand/name of your drive, not the name you selected)
Go to Partition and you'll see a drop down that says "current" and change that to "1 partition". The name it and select the format as "Mac OS Extended (Journaled)". Hit "options" and make sure its GUID partition table and then hit apply and BOOM you can add to your drive.
Check out this video if you need more help/a walk through, its for time machine but it does the same setup i just explained and might be better than following my instructions. Refer to time 1:42 - 3:20 for the walkthrough.