This is quite easy. We'll configure your existing hard drive to be a master drive and your new one to be a slave drive. Here's how:
1. Turn off your computer.
2. Pull all of the cables out of the back, including the power chord.
3. Get a screwdriver and open the case.
4. Have a look around inside and look for a flat (thin) but wide grey cable going from your motherboard (the big circuit board at the bottom of your case where everything plugs in to) to your existing hard drive.
5. Make sure there is a spare plug on the cable. One of the plugs will be going into your existing hard drive. Also make sure that there is space in the metal rack where the hard drives go, and that there are screw holes in the rack.
6. Unscrew the hard drive and unplug the IDE cable (the flat grey cable) and the power cable, and remove your existing hard drive.
7. At the same end where the cable went in there will be a series of pins. These are called 'jumper' pins and they're used to determine whether or not the device is master or slave. Two of the pins should be covered with a thin bit of plastic that joins them together - this is called a jumper. (see the URL in the sources section below).
8. Look on the case of the hard drive for a picture of which pins to cover to make the hard drive a 'master' drive. Remove the jumper and put it back on the appropriate pins. Use a pair of tweazers if it helps.
9. Look at your new hard drive and configure it as slave (using the jumper pins, just like in step 8).
10. Place both hard drives back into the computer in the metal racks, and remember which is which (maybe use some sticky tape temporarily to mark one of them). Screw them both into the racks.
11. Attach the master hard drive, your old hard drive, to the very end of the IDE cable (the flat grey cable).
12. Attach the new hard drive to the middle plug in the cable. (see the URL in the sources section).
13. Attach power cables to both hard drives.
14. Put your computer case back on, but don't screw it in yet.
15. Plug in your power, monitor and keyboard and turn the computer on. Hold down the 'delete' key on your keyboard immediately. This will take you to your computer 'bios' screen.
16. Select the first option in the menu. This should list the hard drives that have been detected. If both hard drives are in the list then you're done. Screw the case on properly. If not, double check the jumper settings, the IDE cable and the power cable. If you can't get it to work, DONT PANIC! Just take your computer to your local computer store and ask for help.