Flip through the table of contents to get a feel for the subject (biology, physiology, chemistry, etc.). Skim through each chapter and section of the book to familiarize yourself with the material.
Copy down each of the chapter headings directly from the table of contents. Highlight each of the chapter headings to make them easily identifiable. Write a short description of the chapter's subject and what topics are covered. Write down the major subheadings beneath the chapter titles and describe the topics covered. Leave about a half page of space for each chapter to provide room for later notes.
Go through each chapter individually and look for material offset from the text, usually set within a colored box. These contain information directly connected to your science curriculum. Copy this information down in the notes section you left underneath each chapter heading. Use this space to copy down any other important information you come across as you go through each chapter. In addition to relevant textbook information, also jot down anything important on the topic that is discussed in class. Incorporating your science lectures into your textbook outline will help you retain the information more easily.
Write lists of major vocabulary words and definitions in the appropriate chapter areas. Copy diagrams for biological systems and types of organisms. Use mnemonic devices to remember important classifications (like "ROYGBIV" for the colors of the rainbow). Science uses technical phrases for clarity of classifications, but they can sound fairly indistinguishable to the untrained ear. Utilize memorization tricks to help you keep hold of more advanced scientific processes. Record these in the appropriate notes section for easy categorization.