Read the table of contents and introduction at the beginning of every chapter, then read the main sections to get a general idea of what each chapter is about. Also read any summaries at the end of each chapter.
Put the textbook aside after reading each section and take notes on new knowledge you have just gained. Then, using your textbook again, add in any other important points you think you missed to your notes. Write down any unfamiliar words or chapter points.
As you read through the textbook, ask yourself the types of questions you think your professor may ask. If there are practice questions available, try answering them. Pay special attention to pictures, graphs, tables, and bold words. If there are study aids included, utilize them. As the semester goes on, review your notes and the textbook so you don't forget anything you have learned.