Lets start with the basics. If you buy the Movie / TV Show from iTunes, you'll have no problem except a lighter wallet. But say, you get it from a different place say a DVD or downloaded from somewhere else. You first need to convert your movie to .mp4 the file format which is pretty much one of the only files iTunes accepts. For this I present to you -
1. iSkysoft iMedia Convertor - While it is a bit pricey at $49.99, it is an amazing application which allows you to convert to pretty much any format. It has predefined formats for specific devices such as iPad, Mac, Youtube, HD, Apple TV, iPhone, iPod and several others in addition to letting you make your own custom formats based on required video resolution, etc. You can also download a free trial version from here. A few free alternatives applications for the same purpose are Handbrake and iSquint.
Once converted, the Movies / TV Shows can now simply be dragged to iTunes where they will automatically import themselves into the Movies tab of iTunes. Now you can simply leave them there untouched. Or you could add artwork and make them look like above. To do so, select the Movie / TV Show, press command-I, go to the artwork tab, drag an image in there and click okay.
Now, if they're TV shows do you really want them in your Movies tab ? No. So you select one or all the episodes, press command-I, go to the options tab and change the media kind to "TV Show". If you wish to, go to the videos tab of the same window and type the name and season of the TV Show. Episodes of the same TV Show name get grouped together.
Next Up : TV Shows Application ( For educational purposes only )
TV Shows Application - This is an amazing application, free of cost. You simply choose from a MASSIVE list of TV Shows and it will download the newest episode of the show from torrents or any other source of your choice as soon as it is released. Also gives you the option to download HD versions of the show by default.
The third application -
VLC - Even if you have problems opening or playing the video with ANY other media player, which you most likely will, you won't have one with VLC. It is by far the most amazing application I have ever used for video playback. Its also quite useful if you're simply too lazy or if you don't have the patience to convert the video before watching. It can play ANY kind of video no matter what. I have one simple rule: If it doesn't work on VLC, its not a video.