The first job is to communicate (normally with the aid of a handout) an outline of the assigned reading. Where the selection is organized as an argument, you will present an outline of the argument (see the instructions on argument analysis).
Outlining is a discipline often neglected after primary school or junior high. Outlining obliges you to overcome the tendency to grab what is of most interest to you and simply to make your comment on that. Outlining obliges you to stay with—and reproduce in a way you can reasonably expect the author to approve—the way the piece unfolds from start to finish. One way (time-consuming, but worth it for some texts) is to summarize the meaning of each paragraph, then look for meaningful groups of paragraphs, and groups of groups, etc., until a structure of the whole emerges. Beware of relying on editorial additions, which may neglect important dimensions of what the author is trying to do. Of course, you should not normally expect to present much of the data gathered by detailed argument analysis or outlining.
Take 15-20 minutes, leaving the last 5 (of 15) to 10 (of 20) for discussion. Offer some response of your own to the selection, and pose some questions to start discussion. You need to empower the students to discuss. Coming up with good questions is one of the most important things you can do; please take the time to do this thoughtfully.
You are the teacher during this time. Conduct yourself accordingly.