Getting movies to "fit" on a TV screen is actually more complex a process than some may think. The core of the issue lies in aspect ratio--the length of the screen measured against its width. Television sets don't always match up to the precise size of a given movie. To solve it, you can either watch a moving in letterbox format--with black bars at the top and the bottom of the screen--or a "pan-and-scan" version that cuts off part of the image to fill the whole screen. Which one you choose depends on taste and on the movie in question, but there are a number of tools to help you do it, regardless of what kind of TV you have.
Understand the difference between widescreen and pan-and-scan. Movie purists--including every director under the sun--believe that widescreen is the only proper way to show a movie. It preserves the director's original vision and allows the entire image to be seen. Pan-and-scan versions fill the screen, but often at the expense of camera placement and focus. The film is essentially re-edited, with the camera moving to center on parts of the truncated image and cuts made to accommodate previously unbroken shots. Whether or not that bothers you is purely a matter of personal taste, but you should be aware of the distinction before proceeding.
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Check the aspect ratio of your TV. It should be listed on your owner's manual. Older tube TVs almost always have 4:3 screens, which work for some kinds of films (usually older) but not for others. Newer flat screen TVs have screens in a wider 16:9 ratio, which better accommodates widescreen movies. Many flat screens also posses aspect adjusters, able to change the aspect ratio of the film to accommodate personal taste.
Decide if you prefer to see movies as they were originally filmed or if you would rather fill your entire screen with the image. If you choose the former, you need to accept the notion that you're not using all of your screen with some films (there will be black bars sometimes). If you choose the latter, you need to accept the notion that parts of the film may be cut off.
Check the DVD cover for any film you want to watch. It may list "widescreen" or "full screen" (pan and scan) on the cover, alerting you to the format it uses. (Not every movie offers both options, however). In addition, you can check the back to note its aspect ratio (it's usually included in the small print towards the bottom) , and compare it to the aspect ratio of your TV. A widescreen film often includes statements along the lines of "preserves its original aspect ratio."
Scan TV listings for movies that you want to see. They should tell you whether the films are broadcast in widescreen or full screen/pan-and-scan. Big networks and stations such as TNT usually broadcast movies in pan and scan, while specialty movie channels like TCM, IFC and SciFi often play them in widescreen.
If you have an older 4:3 aspect TV, select only those movies that match your preference. If you prefer pan-and-scan, select only full- screen movies to watch. Otherwise, watch only widescreen movies. You won't be able to adjust the image once you play the movie, so be selective beforehand.
If you have a 16:9 TV, you have more options. Use the aspect ratio button on your TV's remote to adjust the size of the image. Pan-and-scan fans can use it to fill the entire screen with an image, while widescreen fans can keep it on the normal setting to preserve the film the way it was shot. They can also stretch the image of older films shot in 4:3 to fill the entire screen (eliminating the black bars to the left and the right).