I am going to assume that the HDD is already installed in the computer. If the HDD is telling you it is Unallocated, the drive is not ready for use because it has no Partitions.
Go to Start, then right click Computer and select Manage.
This will open the Computer Management window.
Look at the Menu tree to the left and click Disk Management.
You should see your drives listed.
Right click the drive that is Unallocated and select Create New Volume
After you see the New Volume listed, right click it and select Format.
The drive is now ready for use.
You can also right click any drive, scroll down and click Properties
You can change the name of the drive there.