Take notes. Write down important aspects of the lesson, including steps to solving problems that the instructor completes on the board. Writing things down will help you retain the information better and provide information to review later.
Plan time for algebra review and study. Take time when you're not in class to complete homework. If you are not assigned any homework, review your notes and solve some problems from your textbook.
Practice as much as required to retain what you were taught in class. After you have finished your assigned work, put your notes away and try to do more problems on your own. That will help you memorize the algebra concepts. Redo problems that give you trouble until they become easy to finish. If you do not have an algebra class every day, choose to practice in between classes. Seek out free algebra resources and study guides online to help with algebra practice.
Get help if you need it. Do not be afraid to ask your instructor for individual attention after class. Consider studying with classmates or hiring a tutor if necessary.