Hi!
First and most importantly, you'll need to apply and for a wildlife keeping permit. This permit allows you to buy and keep many species of native wildlife, including snakes. I'm not to sure how much it is in NSW, but if it's anything like Victoria's system, you'll need to pay a yearly fee to keep your permit.
As for purchasing a snake, I think that some pet shops are okay to buy from, I hear good things about 'Kellyville pets'. But you don't want to buy from places like 'Best friends' or 'Petbarn', ideally look for breeders as bl69aze said, or look for specialist pet stores that deals almost exclusively with reptiles. If it's a good store, they'll be able to help you out with questions.
As for what species, you've got a ton of choice
If you were wanting something on the smaller side, Antaresia is a good genus, and has your children's/spotted/pygmy pythons. Or you've got your carpet pythons, which get much bigger, the biggest being a coastal carpet python, (longest) the smallest is a jungle carpet python. Of course there are many more snakes out there, but these are what i'd recommend for a newbie, and are what's most popular. Just look into what species interests you the most, and go from there.
And finally I'd recommend getting the book "Keeping and breeding Australian pythons" by Mike Swan, it's a little pricey but it's got great information and is very much worth it if you are serious about getting a snake