What do I need so I can watch TV on my computer?

There are online channels you can watch like: http://wwitv.com
All you should need is a video card, which you probably already have and probably Flash which you may also have. If you do not have it then you can download it free on http://www.tucows.com
http://www.hulu.com is more popular than WWITV and has a lot of the more popular shows. WWITV has all the international shows.
Now the other way is with a tuner.
A tuner specifically designed to interface with your PC. These devices are available with a variety of interfaces and feature sets offering great compatibility with just about any personal computer (PC).
The basic principle of a PC TV tuner is that you connect the device to your computer, and then you connect your cable or antenna television source to the device, just like you would connect to any typical TV.

There is an interface to turn just about any available connection on your PC into a TV source, including: PCI, USB, Cardbus, PCI Express, and FireWire. So, it doesn’t matter whether you have a desktop or laptop, or if you are running Windows, Linux, or a Mac. You can find standard (NTSC/PAL) cable or HDTV tuners configured in single or dual tuner arrangements.

All of the tuning is done with a combination of hardware and software, so quality will vary from one tuner to another based on the quality of the tuner components and the design of the application and drivers. Watching television is the easy part, but the great thing about these tuners is the ability to record.

Higher-end cards will provide better video quality during live television playback, and in general will offer more options and capabilities when it comes to recording and saving programs to your hard drive. Having a dual-tuner card allows you to watch one show while recording another, or even record two shows at the same time. Even if you don’t have a dual-tuner card, you can create a similar setup by installing multiple TV tuners into one computer.

All of this, especially the recording and encoding, requires some reasonable computing power. Many times just meeting the minimum specifications published by the manufacturer will leave you less than impressed. In addition to a fast processor, you’ll need enough memory and a hard drive that is not only fast enough to keep up, but large enough to hold all of your recordings.

One big upside to this method of getting television onto your computer is the wide array of choices, interfaces, and price ranges. You can be up and running on a basic tuner for around $25, and since they are also available with USB and FireWire connections, your installation might take just a minute or so and doesn’t have to involve opening your computer case. Other strong points of this method are the recording capabilities, potential for high quality audio and video, portability when used with a laptop, and the ability to install multiple tuners in one system.

A possible downside to this method is that you need a computer with the horsepower to keep up with the demands of your tuner or tuners. Just watching cable TV might not stress a system that is even several years old, but trying to watch an HD broadcast while recording from another tuner might best be done on a more modern computer. Another potential downside is also related to HD, and that is that most HDTV tuners are for over-the-air high definition broadcasts only. There are tuners that can access HD digital cable, but the vast majority of HD tuners will require an antenna and also that you be physically located in the vicinity of an over-the-air HD broadcast.

Most of the TV I watch is on the internet, much of it is free, and there's a lot out there.

It depends on if you want to watch TV over the internet, or if you'd like to connect your computer to your cable or satellite connection. If you'd like to watch it over the internet there are many options, though these are what  use:

http://www.hulu.com - They have a well organized site that you can use to view many different tv shows (and some movies) shortly after they're aired.

http://www.netflix.com - If you have a netflix subscription you can steam quite a few tv shows that they store on their servers. These typically aren't posted the moment they are aired, though you can find several seasons of many shows here.

http://www.nbc.com http://www.fox.com etc - Many tv stations will let you watch their shows, sometimes in high definition, for free through their website. The commercials are very minimal which is also nice

If you want to watch your cable or satellite shows on your computer:
As someone mentioned a TV tuner is one option, here's a good one: http://www.shopping.hp.com

You just plug that into your usb port, and the coax cable from your cable line into the other end. It downloads your program guide and you can set it up like a typical DVR.

I hops this helps

TV Tuner

I found a some things for you to check out for watching tv on your computer.
TV Neto  http://tvneto.com/download/?utm_source=yahoo&utm_medium=cpc&utm_content=free_tv&utm_campaign=avi

This one is information from Microsoft Windows- I'm not sure that is what you have though but am including it for you just in case.
http://windows.microsoft.com/en-us/windows-vista/How-do-I-watch-or-record-TV-on-my-computer

Here is another one to check out.   http://streamdirect-tvpc.com/2/

This has great information on watching ABC, NBC, CBS, CNN< FOX online free.
http://www.articlealley.com/article_1296698_11.html

Hula TV has many shows, channels to watch online    http://www.hulu.com/