If you wish to participate in a course to learn about the subject but not be required to earn a letter grade, you can audit the course. You will receive a grade of satisfactory audit or unsatisfactory audit but no credit.
Payment is the same as for a credit class.
Professors will expect you to attend class and to be prepared. If you think you already know the material in a course you are required to take, ask the department chair for a challenge exam. Or, of course, you may be told you have to take the course anyway. Exceptions are lab courses wherein one credit hour equals two to four class hours per week. The average number of credit hours for a one-semester course is three.