Plug it into the pc (with the pc turned off), and then boot and enter BIOS, set it as a lower boot option than your primary hard drive. You can now boot the pc and it will boot from your main hard drive so you can reformat the drive.
Here is how to completely wipe a drive: click here. Obviously since you are wiping the drive you will lose all data on it, make sure that anything you may want has been copied off first. You must also be careful that you wipe the correct drive, make sure you triple check before making any changes!
Once you have done all of that it will need to be formatted to be usable, follow this guide: click here
Once all of that is done you will be able to use the drive as extra storage for your pc. If you have any issues just post them here.