A few years ago, before SATA, adding a second hard drive was made more complicated due to the fact that PATA (also known as EIDE) required a master/slave relationship. If you wanted to attach two hard disks to the same channel, one had to be set to be the master, the other the slave.
With modern SATA, which is a serial bus instead of the older parallel EIDE bus, things are simpler. It's a point-to-point bus which can have just one device connected on each 'channel'. That means no jumpers to set and no fear of getting it wrong.
With SATA, all you need is a free SATA port on the motherboard, a spare bay in which to mount your second hard drive, and a SATA cable to connect the two.
Since there's no master / slave setup, there is no 'master' hard drive. Every drive is essentially the same, but this means you need to tell the motherboard's BIOS which drive to boot from.
This is easy on most motherboards. Simply turn on your PC, press the Del key a few times (or look for a message telling which key to press to enter Setup) and the BIOS should appear.
Look for a 'boot order' or 'boot priority' menu and you can then set the appropriate drive to be top of the list. Usually, the BIOS will attempt to boot from each drive in order, which means you can set your DVD drive to first and the hard drive on which Windows is installed second.
That way, you can easily boot from a CD or DVD without altering the BIOS settings, but it might add a few seconds to the boot time while the optical drive is checked for a bootable disc.
It's wise to check your motherboard manual and connect a new drive to a matching SATA port (one running at the same speed as your drive). For example, a modern SSD is likely to support the SATA III standard, also known as SATA 6.0Gb/s.
Your motherboard might have some SATA II ports (running at 3Gb/s) and some SATA III ports. On Intel boards, for example, SATA II ports tend to be black; SATA III ports are blue.