If you're going to work in a licensed premises (that is, a place that sells alcohol), you must complete competency training in the Responsible Service of Alcohol (RSA). If you don't have valid evidence that you've completed the RSA training required for working in a licensed premises, you could be given a heavy fine.
You'll need to be trained by a Registered Training Organisation (RTO). You can find an RTO online through the Liquor & Gaming NSW website. Once you've undertaken your training, you'll need to Apply for an RSA or RCG competency card.