You've found the perfect job (or internship, or bursary) with a government department. Applying to government positions is a complicated process, so here's a guide on how to apply to government jobs.
How to Apply to Government Postings - Jobs, Internships, Bursaries
So you've found a government job or internship or bursary you're interested in and want to apply for it. The first thing you need to do is note down the reference number as stated in the advert.
Next, you need to get your hands on a Z83 form. The Z83 is the "Application for Employment" form and is designed to assist government departments in choosing the right person for the position. You can download one here, or get a copy from your nearest government department.