A astronomy-oriented units/scientific notation website is found here.
A Scientific Notation reference with practice problems is here
Through a Google search I found a metric system reference that is reasonable (some information is in Appendix A of your textbook): The International System of Units: A guide for teachers, students and professionals
I also found a link that should help you with any Basic Math Anxiety (although I will try to go over in class things that you will need/have not understood). This has sections on both Scientific Notation and the Metric Syste m: Basic Math Concepts and Fundamentals
This link doesn't seem to cover the metric system per se, but has a section on Scientific Notation and another on Ratios under the Algebra section. I didn't come up with the name: Math for Morons Like Us