You don't really need anything to install a hard drive. Perhaps a screw driver to screw the hard drive into the bay. You obviously need a second hard drive and SATA data/power cables, or an IDE cable and a 4 pin power connector. You'll also need a spare SATA/IDE port on your motherboard to plug it in.
If you're unaware, SATA cables are the L-shaped ones, the little one is data, the big one is power. IDE are the wide ribbon cables with lots of pins, 4-pin connectors are self explanatory.
1. Turn off your computer and remove the power plug from the back before opening up the case.
2. Ground yourself by touching the case, or preferably with a grounding wrist strap (you should be able to grab one of these from a hardware store).
3. Place the hard drive into the appropriate bay, leave a gap between the two drives if possible, and screw it into place with the screws that hopefully came with the drive.
4. After that, simply plug the SATA/IDE cables into your motherboard and the back of the hard drive and plug the SATA power/4-pin connector into the hard drive.
5. Close the case, plug the computer back into the mains and you should be good to go.
EDIT: Being foolish, I forgot to mention that you will need to format your disk drive. I'll assume you're running windows, in which case you'll want to go to Control Panel > Administrative Tools > Computer Management > Storage > Disk Management, then right click the second hard drive you just installed (C: is probably the name of your original hard drive) and click format, set the disk name to blank or give it a name, make sure the chosen file system is NTFS and then check the "Quick Format" box, before hitting okay and giving it a while to format. It shouldn't take very long, a couple of minutes at most.
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