You usually have a little manual that come along when you buy a hard disk, you can refer to it. Or try these procedures:
Their is a cable a SATA (small one) and an IDE (big one) that you need to connect to the hard disk and into the motherboard and also the power cable to power your hard disk. Make sure you power off the unit when you do this. Once you have connected everything and then power on you press F2 or Del on you keyboard to get into the BIOS - Please refer to your motherboard manual for this. You should see the new hard disk.