In your introduction, clearly identify your subject--in this case, the group you are classifying. If you have narrowed your subject in any way (for example, types of bad drivers, rock guitarists, orannoying moviegoers), you should make this clear from the start.
In your introduction, you may also want to provide some specific descriptive or informative details to attract the interest of your readers and suggest the purpose of the essay.
Finally, be sure to include a thesis sentence (usually at the end of the introduction) that briefly identifies the main types or approaches you are about to examine.
Here's an example of a short but effective introductory paragraph to a classification essay:
It's a warm evening in July, and all across the country Americans are gathering to watch a game of professional baseball. Armed with hot dogs and cold drinks, they stroll to their seats, some in grand stadiums, others in cozy minor-league parks. But no matter where the game is played, you will find the same three types of baseball fan: the Party Rooter, the Sunshine Supporter, and the Diehard Fan.
Notice how this introduction creates certain expectations. The specific details provide a setting(a ballpark on "a warm evening in July") in which we expect to see the various fans described. In addition, the labels assigned to these fans (the Party Rooter, the Sunshine Supporter, and the Diehard Fan) lead us to expect descriptions of each type in the order they're given. A good writer will go on to fulfill these expectations in the body of the essay.
Begin each body paragraph with a topic sentence that identifies a particular type or approach. Then go on to describe or illustrate each type with specific details.
Arrange your body paragraphs in whatever order strikes you as clear and logical--say, from the least effective approach to the most effective, or from the most common type to the least familiar (or the other way around)....