Read the paragraph once, without stopping, to see if the meaning is clear. If so, you can move on to the next paragraph.
If the meaning is not clear, reread the paragraph, looking for repeating words or phrases. If the author uses the same terms several times in a paragraph, that is typically a clue as to the primary focus of that paragraph.
Focus on the first sentence in the paragraph. This is generally the sentence that sets the topic for the paragraph. The rest of the sentences support the first one by supplying additional information.
Review the last sentence in a paragraph; since it typically summarizes the rest of the paragraph, this will also help you discern the main idea of the paragraph.