You will need a TV Tuner expansion card for your computer. If you buy a TV Tuner expansion card and install it, and connect an antenna or cable TV cable, you will be able to watch TV shows on your computer monitor while using your computer. With some TV tuner cards, you can also record TV for watching it later, or even hook up your VCR and record VHS tapes and home videos to put on DVD. This is a more complicated topic and instructions should come with the TV Tuner expansion card you purchase.
TV tuner cards usually run in the range of $20-75 and will fit in most computers. You can buy them online and at computer or electronics stores. Installation is fairly simple and the instructions and software are included (unless you buy a used one)