Depending on the age of your system you may have an ESDI or early IDE disk vs a SATA drive. You'll need to open the system up an do some research to find out what you have.
If you have an ESDI or early IDE drive, you'll need a PCI HD controller that fits into your system in a vacant slot.
SATA drives are much easier to deal with. You could connect it internally if the system has a spare SATA port and power connection. You may need to buy a SATA cable set for the drive. If the system doesn't have a free port or place to hold the drive you'll need to get an external USB case to place the HD into.
The last issue is the OS you are using in the working system. You may need to upgrade it if its older than the system you are pulling the drive from.