If you dont know what to do, try and work out where you would like to be in 5-10 years.
Your two choices are basically
1. Go to uni
2. start working
Going to uni is easy, you dont even have to pick a particular career path just yet, do a bachelor of science or arts and pick subjects in a bit of everything. When you find something you like, major in it and when you finish your degree, get a job doing it.
2. starting work is also pretty easy. Pick a job that pays alot and you wouldnt mind doing for the next,say 10 years and there ya go. A trade is a good way to make lots of money without going to uni, consider an apprenticeship in something you enjoy[e.g if you dont mind cooking food, become a chef]
If neither of those two works out, cosider joining the defense force. They provide some great jobs with great experiences and decent pay. Good training, and you will be a different person when you leave.
Above all else, dont just decide to fuck around and do nothing for a bit after school. If you are going to work, drop your shitty after school job at Coles or whatever and get into contact with some people. IF you just fuck around and party, go out drinking and so on, you will realize money quickly becomes an issue and you aren't even working towards any goals. Before you know it all your friends are finishing their degrees and landing awesome jobs, and your other friends are making deposits on houses because they have been raking in the cash for a bit now.
If you want to go to uni, you should probably be deciding that in the next few months and working towards it.