When you were learning to read, you started at the beginning, sounding each word out until you read a sentence. Fiction books are stories, and sometimes there are pictures. They are suited to a specific grade level, and you can usually tell by the size of the pictures and the length of the paragraphs. Once you hit "chapter books," you think you've hit the reading big-time.
It's very different reading a science textbook. It is not arranged like a story, and usually if you start with the first sentence, you'll be distracted by the second paragraph. Also, pictures are usually not present just to look nice or to indicate the reading level of the book. The pictures in a science book are VERY important. Knowing this will help you when reading.
By following these simple steps, you will find your life in science class much, much easier.