The classification essay uses one of the key skills required not only for writing other essay types but for living life generally: the ability to sort ideas and things into categories. Fortunately, most students already have the skills required to write a classification essay, since almost all students are studying in a field that requires them to sort out concepts. For this reason, the classification essay, once a student has some control of academic writing, should come fairly easily.
There are two steps involved in planning a classification essay. The first step is to decide what kind of thing you want to classify. If you have an open-ended assignment (such as "Write a classification essay about something you encounter every day"), it's best to pick a topic that you know something about. If you're into computers, write about computers. If you're into art, write about different styles of painting. The most important thing to remember when choosing a classification essay topic is that it should be representative of a distinct category. For example, computers are types of things that belong in a category of their own. A classification essay about types of technology, meanwhile, would probably be too broad for a typical English class essay.
Choosing three items within your main category (which is step two) will allow you to write three body paragraphs, an introduction, and a conclusion, resulting in a clean five paragraph essay. If, for instance, you chose to write about three types of computers, your essay will contain three body paragraphs, each explaining the defining characteristics of a different computer type (for instance, Macs, PCs, and Suns). The thesis statement in your introduction paragraph will simply state that there are three types of computers commonly used and that these have similarities typical of their class as well as differences that make them different within their class. Since similarities and differences are being compared, it is important to have a grasp of the comparison essay before writing a classification essay, which is why most English professors will assign the comparison essay early in the semester.