If the classes you wish to choose clash, you will need to select an alternative class. Go to My Class Timetable and swap into a different class.
Prior to classes starting, new places become available in full classes from time to time. If you're keen on a certain class time, check My Class Timetable regularly to see if a new place has opened up.
If your class timetable shows you have successfully registered with a clash, check your timetable week by week. It is likely that you have classes that alternate either on a weekly, fortnightly or ad hoc basis that may appear, at first glance, as a clash. Check the guide on how to view your timetable week by week.
Still have a clash? You will need to change one of the units you're enrolled in. Check your course planner to choose another unit and then check the timetable planner to see if the timetable will suit.