You've asked a pretty big question here, and honestly this all comes down to what sort of work do you actually want to do with reptiles?
Do you want to work with reptiles in captivity such as at zoos or wildlife parks?
If so undertaking the animal studies and captive animal management courses offered at tafe as mentioned by Bl69aze above is what you want to aim for. Personally though I would highly recommend initially doing these courses part time through school and making sure you graduate high school before focusing on it full time. As alluded to above, being a zoo keeper is a highly sought after and competitive field to try and break into, and there is no guarantee you will actually be able to find a work in this field. By finishing high school you're keeping your options open if the zoo keeping profession does't pan out. As they say, don't stick all of your eggs in one basket!
Alternatively, do you want to undertake research into reptiles and be a herpetologist?
If this is your preferred option then you need to complete high school and then look into undertaking a Bachelor of Science majoring in ecology or zoology (or something of the like). This option is certainly the longer haul of the two and is potentially even harder of a field to get into (although I found it rather easy), but for myself personally at least, it is the most rewarding and interesting. There is nothing quite like being the first person to discover something that has never been known before. And of course this isn't the only way into this field, but it is the easiest and most direct way.
Regardless of which of the above options you choose, the single best piece of advice I can offer you is to VOLUNTEER! Volunteering can never be underestimated and has three crucial benefits for you:
1) You get to gain direct experience and up-skill yourself in the field of your choosing. There is no better way to learn than by actually doing!
2) You create connections and network with people already in the industry you would like to enter. Never underestimate how important doing this is! It can be the difference between getting a job or not.
3) You get to find out if you actually enjoy the field of work and if you would like to continue. There's no point starting a job in a new field only to find out a month in that you actually hate it!
Hopefully this advice is helpful for you, and good luck with it all.
Cheers, Cameron