For 12 credit hours, you'll need to spend at least 36 hrs. That depends on your schedule. You should attend all of your classes. If you miss too much you could fail. Each instructor has different rules about attendance.
Some instructors take attendance every day and others never take attendance.
Find out your instructors' rules. This form needs to be taken to the Registrars's Office. You will have to pay for the class at the Cashier's Office only if you are adding a class. Look in the course Schedule book for deadlines for dropping and adding. Last day to Withdraw-End of 12 the week of classes.