The following is for elementary classes such as calculus.
A concerned student should (1) attend the tutoring sessions in Reiss (beginning after about the first two weeks of class), (2) work with a classmate or two in an organized way - so that some of you can help the others thereby learning it better yourselves, (3) practice problems from the text (or other sources, e.g., other books) comparing answers with each other and with those for which answers are given. After you have tried these methods, if you still have questions, then come to see me!
For more advanced classes, try methods (2) and (3) above and then come to see me.
General information, including for various courses, on the index (classroom) page .