I really want to take this beginning weightlifting class but I need to get in shape for it first. What types of exercises should I do to get prepared?

This is a little like saying I need to go in the pool and get used to the water before I take a class that teaches swimming. The purpose of a beginning class in fitness is to help people start their fitness training and learn how to exercise safely and effectively. Fitness instructors are aware that their participants may not be in the best physical shape, but that is exactly why such classes are offered.  It is a fitness instructors job to coach and introduce people to the various moves and different pieces of equipment. Instructors are also there to support you and answer any questions or address any concerns so that participants have a productive, positive experience. Plus once you get into better shape, you will be able to participate in more advanced classes with confidence.