This is often difficult for weaker readers, who want to give a long, detailed explanation of the site. I stop them and remind them that finding the main idea requires seeing the big picture of what they read. This can be very challenging, as the way the main idea is expressed will vary from writer to writer. Since Web sites are frequently organized around discrete chunks of information, finding the main idea requires students to do some higher-order thinking and synthesizing of ideas. Many students need to observe you finding the main idea of a text to understand the thinking process.
Some students do well with identifying key ideas as a preliminary step to finding the main idea.
If a Web page has specific topic areas, I teach students to examine these topics for clues to the main idea of the text. I also have them look for signal words in the titles such as Introduction or Overview from the site home page, as well as signals words within the text such as therefore, in conclusion, and in summary. Once students have grasped the main idea of a site or a piece of text within the site, we begin to practice summarizing what was read. I want students to start with the topic of the reading and add only the most important supporting details. I find it helps students' thinking processes to have them discuss how they reached their decisions of what to include and what to leave out.