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After you install the card successfully, you need to hook it up to an aerial/satellite/cable feed (i.e depending on the card type you buy) to pick up channels - just as you would with any other TV, satellite decoder, etc.
The card should come with software for your PC allowing you to scan for channels and watch them. There are 3rd party programmes (some free, others you pay for) which you can use as well with most cards which may be better than the bundled software.
I have a DVB-T type TV card (picks up standard Freeview over an aerial). I didn't like the bundled software and so use MyTheatre instead: