|First you have not told us anything about your PC.|
Is it a laptop or a desktop?
What is its make and model?
If it is a generic clone then what make of motherboard has it got?
If it is a desktop then what kind of hard drives are they? SATA or IDE?
Without us knowing what your PC spec is we cannot give you a well informed answer.
Having said all that. I will now assume you have a desktop with IDE hard drives.
Now assuming that you currently have one hard drive and one CD/DVD drive.
Also assuming that the hard drive is connected on IDE 0 as a master and the CD/DVD is connected on IDE 1 as a master.
These assumptions therefore mean that you have a spare position on both IDE 0 and IDE 1.
To connect your second hard drive you have to jumper it as a slave and connect it to the spare cable connector on IDE 0 OR IDE 1 and connect power to it using a spare molex connector.
Once you have connected the hard drive, start your PC and go into your BIOS by tapping the F2 or Del key as your PC is booting.
Once in the BIOS set the corresponding IDE setting in the BIOS to auto so that your additional hard drive can be detected in BIOS. Save and exit from the BIOS.
Your PC should now reboot and detect the newly added hard drive. Once up and running, in windows, open windows explorer. If the second hard drive was already formatted then you should be able to start using it. If not then you will first need to partition and format the newly added hard drive. Once that is done you should be able to use it.
☺ When everything else fails, read the instructions.