Start with a script and define how long it's going to be. Even if there's not going to be any dialogue, you still need the framework of a plot that will carry the action through from beginning to end. Example: Boy gets new dog, boy loses dog in park, boy finds dog at the ice cream shop. If you have decided that your movie is going to be 15 minutes long, this means that you will have five minutes each to devote to the beginning, middle and end.
Decide who the characters in your film will be and how many scenes you will need to tell the story. For instance, will there be humans interacting with the animal? Will there be other animals interacting with the animal? Will all of the action take place in one location or will there be multiple settings (i.e., the animal shelter, the park, the ice cream parlor)? Keep in mind that the shorter your film is, the smaller the number of players and the fewer locales you should use. Otherwise, things are going to get much too confusing for your audience.
Determine whose point of view will prevail. In the example of the boy who loses the dog, the first assumption might be that the story is told/seen through the boy's point of view. But what if it were the point of view of the dog? For instance, is he frightened to be separated from his new owner, or has he already found the kid to be a complete annoyance who he can't get away from fast enough? Or maybe the dog was rescued from the local animal shelter but has an unwavering commitment to find his way back to his previous owner. Maybe the viewpoint is even that of a squirrel who has been sitting up in a tree and watching all of this mania ensue. You should also decide at this time whether the animal is going to just be a regular animal or whether he will exhibit anthropomorphic traits such as talking like a human.
Decide on a genre for your animal movie. The genre you choose---comedy, drama, horror, fantasy---will then dictate elements such as the camera angles, the lighting, the special effects and the music. If you decide to bypass the entire idea of a script and are more interested in the subject of nature as a documentary, this is an acceptable course, too; simply assemble all your video footage of critters who wander into your backyard and then write the appropriate voice-over commentary on what they're doing.
Decide whether you're going to use real animals in your movie. Unless you have smart pets that always do tricks on command or you're willing to shoot lots and lots of video to get something clever to use, you need to be aware that animals---just like children---will rarely do exactly what you want them to do when you bring out a camera. For that matter, you should also decide whether you want to use real humans in your movie. Considering that this is an amateur production, the cast you have available to you who will be willing to work all day for free and bring their own costumes are probably your family, your friends and---if you're a student---kids from school. If you're not used to assuming a strong leadership position as director and telling others what to do, filmmaking might not be the best place to start doing this.
Explore the alternatives of not using live animals in your film. For instance, a cast of stuffed animals might be able to deliver the same story through stop-motion photography and voice-overs. Are you good at animation or creating dimensional figures on a computer? Websites such as Calico Monkey and Drawing Coach (see Resources) offer tutuorials and links to animation software.
Roll the cameras! Whether your stars are household pets or Beanie Babies, the objective is to have fun and create a story that your audience will enjoy. Incorporate sound effects (see Resources) and music to set the ambience.