Speak to Tennis Victoria or Tennis Australia. They will inform you of when the next Level 0 course will be held. Start there. If coaching is your thing, go for your Level 1's, Level 2's, Level 3's, Level 4's etc. Eventually, you could possibly work your way up to a Master Coach course. This will take time. Even some of the best coaches of professional players are not at Master Coach level. It's very hard to get there.
To get experience while you are doing your Level-standard certificates, I recommend approaching a tennis coaching school for kids. If you have had previous experience, you may be given jobs right away. If you're just starting out, you might share classes for awhile before going it alone. Your very own class for the first time can be daunting. At these schools, you will most likely be instructing young kids, pre-teens and sometimes older kids.
It's not recommended to start coaching until you are out of that "high school" mentality. The last thing you want is one of your own students bullying you while you're supposed to be instructing them.
Coaching is not easy. It takes patience, good organization, clear thoughts and excellent verbal skills. You can work on most of those things but it's essential that you have some of them already firmly tucked under your belt.
Most of the instructors working at the coaching school are young adults who've recently come out of junior ranks. In other words, they are 18 or older. Almost none have much experience apart from the actual practical, hands-on exeprience.
Expect to get paid about $15 - $20 an hour. That's more generous than your average job, but you also get less hours because you can only take a class or coach a person for so long. It's quite physically demanding.
I've been working part-time (that's the nature of the job, unfortunately) for a coaching school since about February. I haven't even got my Level 0's yet, but it's good practical experience. I never in my life thought I'd be working with kids, but here I am. It can be exhausting and testing at times, but also rewarding and fun. All kids are different. You have to take the good with the bad.
I hope some of that helped. If you want to know more about what it's like, feel free to post more questions here.