How do I connect an old external hard drive to an early 2006 iMac using a firewire cable.

The old 2006 iMac supported FW400. FW400 uses a 6-pin cable that looks like this:

Now, I have no idea what Firewire connection is on the drive, but if it also looks like the one on your old iMac, then you need a standard FW400 cable. However, the drive's enclosure may not even support Firewire, but I would have to see the rear of the drive to tell what it did support. For example, if it supports USB, then you could also use a USB cable because the iMac also supports USB.

That is a bare drive. You need an external enclosure for it unless you are replacing the computer's internal drive. You can use most any notebook size enclosure that supports a SATA notebook drive. The least expensive will be USB enclosures. Firewire enclosures are more expensive, usually.