You need to partition it, and then format it.
If you added a new hdd to your system, the procedure is:
1. Click start, then right-click My Computer
2. Click Manage
3. From the tree, chose "Disk Management"
4. You should see your disk in the list bellow, (with other physical storage
devices, such as CD drives).
5. Click on the you new hdd in the list bellow, and then right click on the
empty space on the right, and chose
create a new partition. Wizard will appear, that enables you to specify the
type of a partition, size, and format it.
Best regards,