Answers to your two questions:
but do I need to enrol to make sure I have my spot?
Yes, enrol as soon as you can. You can always withdraw if you accept a 2nd round offer (as long as it is done before the "census date"). You guarantee yourself a spot in the course once you enrol. If you wait and leave it to chance, there's a chance you won't get a spot in it (this means your offer will "lapse").
Also can someone explain what SA-Help is?
Each year all students at universities pay a fee called a Student Services and Amenities Fee (or equivalent). This is usually around the ~$250 mark per year but it varies. This is
separate to HECS-HELP, as HECS-HELP is for your course tuition fees (e.g. price for each subject you do). If you sign up to this, the yearly fee will be added onto your HECS debt, meaning no upfront fees there as well!