Reading primary texts in philosophy can be particularly challenging. Don't think of these texts as something you merely read, but as something you study and analyze. You'll need to set aside more time than you would for reading a novel or ordinary text book.
I highly recommend reading the material more than once. Skim through the assignment the first time, paying special attention to terms that appear often and seem to be important. Underline or highlight those terms; put a check mark by sentences or paragraphs you think you think might be important later. Look closely at how sections (or paragraphs) start and finish. Note section headers and titles as guides to what the section is about. You can read the concluding paragraph(s) carefully now, especially if it contains a summary. You are going for the big picture. Try to get a sense of what the reading assignment is intended to do. What is the author trying to prove? What idea(s) is the author critiquing or challenging?
Now go back and read the assignment more carefully. Study it one section at a time, or even one paragraph at a time. For each section of the text, stop when you finished reading it and ask yourself to summarize the key point of just that section. Go back if you have trouble doing this. If the section or paragraph remains unclear, make note of it (try to say what it is that is making it difficult) and go on. Underline or highlight key terms so you can find easily if you need to go back. (But highlight conservatively else you may loose the benefit of the marking.) Make notes in the margin about a special point the author makes there or about a question you have. If there are conclusions the author is trying to defend, what reasons does he or she offer in defense? What objection do they anticipate, if any?
Your examination of the text is not complete until (1) you can summarize in a few sentences what the assigned text was about (rehearse your answer to the request “Tell the class what this assignment was about”), (2) you can say what parts of the text you found most interesting, and (3) what parts of the text you found most difficult to follow or understand.
Here's more advice about reading (though mostly about non-philosophy books):