Local broadcast television channels use federally regulated call signs as identifiers, so often the URL for the broadcast channel will include the letters from the Federal Communication Commission's assigned call sign. Other station's websites can be found either by typing in the name of the station or with a little creative searching. Note that some cable or satellite-based channels may require proof that you subscribe to the service before you can watch shows online.
Find your local broadcast stations through the Station Index or the FCC website (see links in Resources). Using the Station Index find your television market -- the top 100 markets are listed together and the smaller markets are listed together. If you can't easily find your area by reading down the list, press "Ctrl-F" and type in the name of your area to search the page. Click your area and then click the website link for the station of your choice. If you cannot find your area using the Station Index, open the FCC reception map, enter your ZIP code and the closest stations are listed. Search based on the call sign to find the URL.
Check your cable or satellite provider's website for a channel listing (see links in Resources) to determine the name of the cable channel for which you want a URL. Most cable channel websites are simply the name of the channel with ".COM" appended to the end. Some channels, like TLC, are subsidiaries of other channels -- in this case Discovery -- so when you browse around on the TLC website soon the URL will show "discovery.com" in the address bar.
Watch TV online either through the parent network for a broadcast channel or through one of the websites that contracts with them. For example, if you want to watch a show from ABC you can either find it on ABC.com or Hulu.com, which is partially owned by ABC. Cable and satellite channels may also offer content for free through sites like Hulu or via subscription sites like Netflix or Amazon Prime.