If the professor assigns specific problems, make sure you know how to work every one of those problems and then more. Make sure you know how to work every problem given on a quiz. If you miss a quiz problem, make sure you know what you did wrong so that you will do it right the next time. Some professors give out study guides. If the study guide says that you need to know a specific concept, make sure that you do know it.
Some professors give out practice tests or put old exams on reserve in the library.
Make sure that you know how to do every one of the problems on these exams. Know how to work that problem and that type of problem. Most professors will come right out and tell you almost exactly what will be on the exam, except for the specific numbers or examples. Know the vocabulary of chemistry. In many ways, learning chemistry is like learning another language.