You have asked a very simple and reasonable question. If only Apple's solution were as simple.
The short answer is that you have to get iTunes to recognize and "know" where your home movies are. This depends on the where your home movies are stored (which itself depend on what you used to record those home movies).
*Camcorder movies are generally imported into iMovie.
*Still camera (point-and-shoot, DSLR) movies are generally imported into iPhoto.
*iOS device movies (iphone, ipad, ipod touch) are generally imported into iPhoto when you synch your device using a USB cable. (This doesn't seem to work using wifi sync, you must use the USB cable and treat your iOS device like any other camera.)
For home movies stored in iMovie:
-Create a new "project", and put the clip (or clips) you want into that project.
-When the "project" looks the way you want, export the project to iTunes. During this process you can select the resolution you want (1080p is best if you have HD video, but it also takes up the most disk space).
-Your movie will now appear in iTunes, and is therefore viewable on your Apple TV in the "Movies" category.
-If you like you can edit the movie's metadata in iTunes by clicking on the movie and hitting command-i to show a window with lots of information fields you can edit. You can, for example, change the "genre" to "home movies", which will make your home movies easier to find on your Apple TV if you sort by Genre.
For home movies stored in iPhoto:
-These movies should already be viewable on your Apple TV as long as you have iTunes set up to share your iPhoto library with the Apple TV. (In iTunes, go to Advanced --> Choose Photos to Share. . . http://support.apple.com/kb/HT4362)
-On your Apple TV, go to Computers --> your homeshare library --> Photos. Then go to whatever event or album contains the movie clips. You will see the movie thumbnails distinguished from the photo thumbnails because the movies will have a little "play" icon on them.
-Note that those movies with higher resolution/bit-rate will take longer to stream to your Apple TV.
For example, I have an Apple TV 2. I have 1080p HD home movies both from my iphone 5 and from my DSLR (Canon T2i) stored in my iPhoto events. The iPhone 5 movies stream quickly over my home network. The DSLR movies have a higher bit-rate, and hence they often take WAY too long to stream over my network for them to be enjoyable.