Check what ports your TV has. If its new, it probably has one of the following: VGA (the trapezoidal blue one), DVI (rectangle beige), S-Video (round black), component (three separate ports), or hdmi (small, black, trapezoidal).
Now check what outputs your computer has. If it's a laptop, probably S-video, vga, or maybe DVI. If it's a desktop it could have any of the above.
Once you know which ports you have, find a cable that can connect them. If two of the ports match, you'll just have to get a standard cable, otherwise you will need a cable adapter or the like.