It is quite simple to install a new hard disc. There are two cables which should be connected to the hard disc. The power cable which consists of 4 colored wires seperated from each other and only joined at the plug. Make sure that the red wire on the plug faces the red lining on the data cable (explained below) or in other words, the red wire should face the data cable if it does not have a red lining.
The data cable is a flat and wide cable, usually white in color (other colors are also available) The cable "plug" has a notch on one side which fits into the slot in the hard disc. Make sure you insert the cable gently with the pins in the hard disc aligned with the holes in the cable plug. Fit it firmly and you are done.
Next, place the disc in a casing slot and secure it with screws
For the PC to recognise the disc, you will have to configure the bios. Most likely your bios will be set up to automatically recognise the hard disc. If not, you will have to manually install it. Please read the bios manual to find out how that should be done.
One the disc is recognised in the bios, save it's settings and boot to your OS. In the OS, you will have to format the disc the first time you want to access it. After formatting, the disc is ready for use :)
Hope this helps