You can choose to study part time or full time at UniSA. If you want to study less than full time you simply need to enrol in only the courses that you want to study in that semester. If you need academic advice about which courses to enrol in, please consult your enrolment advice, school or program director.
Please bear in mind that if you study less than 75% of a full time study load you will be considered a part time student which may impact you financially, especially if you claim Centrelink benefits or use the tertiary education discount on Adelaide Metro. If you are an international student you should consider your visa requirements before going part time, more information is available here.
If other responsibilities in your life are impacting your ability to study you may also want to consider speaking to a UniSA Counsellor.