how can I connect an HDMI cable to my Mac?

What kind of ports does your video card have? Mini-displayport? Use a mini-displayport to HDMI adapter. DVI? Use a DVI to HDMI adapter and if you also want audio to your TV (DVI doesn't pass audio) pass the analog stereo line out in the back of your mac pro or figure out how to use the digital optical audio out.

You can get appropriate adapters and cables a places like www.monoprice.com.

Once connected set your Dispplays preferences for the TV to the appropriate resolution (720p, 1080i, 1080p) and refresh rate (60Hz I assume). Might have to click Detect Displays and Gather windows if the TV isn't initially showing anything. I

Also use the Sound system preferences to set the approprate output device.

You are in the Mac Pro discussions so I am assuming you are using a Mac Pro. Mac Pros have a lot more than just firewire. Of particular interest here is the video card you are using. What kind of ports does it have? DVI, mini displayport, what? How are you connecting your mac to your monitor?

If you aren't using a mac pro, then you still need to say what kind of (additional) video connections are supported by your mac. It still probably mini displayport or DVI. But that idea is the same. You need the appropriate adapter. You don't just have a firewire port, firewire is not used for video out on macs, and as far as I know there is no such thing as firewire to HDMI adapter.

So what kind of mac (year, model) are you talking about here?

Then head over there, check specs, you have ThunderBolt video display (a new proprietary for now output interface)

http://www.apple.com/support/imac

Well it is stll try you have more than just firewire ports. FYI, iMacs come with firewire, USB, and as The hatter said above, Thunderbolt video ports. So you therefore need a Thunderbolt (and/or mini-displayport) to HDMI adapter. They are readily available. Here's one at monoprice.com:

Mini DisplayPort | Thunderbolt to HDMI® Adapter w/ Audio Support

Apple sells the Moshi adapter:

Moshi Mini DP to HDMI Adapter with Audio Support

Just to avoid confusion, Thunderbolt and mini-displayport of physically compatible. You can plug a mini-displayport device into a thunderbolt socket (not the other way around) and it will work. So for these purposes a mini-displayport to HDMI adapter is acceptable.