That is a really broad set of questions. If you are studying a degree in australia, many universities have opportunities of taking semester exchanges to other countries.
However, you can also study overseas straight highschool if you want to. As for requirements- it depends where you want to study. And it can cost A LOT. I have a friend that went to oxford for his undergraduate degree. There was minimal help in scholarships, and you are an international student so you have to pay large fees as well as accomodation. On the other hand, I had another friend go to Dartmouth in USA (ivy league university) and they actually had a lot of financial assistance on offer, and she was able to get practically a full scholarship to fund her tuition and accomodation. Both of these friends were very high academic achievers as well (the oxford one got a 100UAI).
You should think about why you want to study overseas, and where you might like to study, and then look at what they have on offer (scholarship wise etc) and what their requirements are. As PwarYuex said, talk to your career's advisor.
Just for informations sake, there are quite a few threads around here on whether there is any real benefit in completing an undergraduate degree overseas- or whether it is just better to study here in australia and then if you want to, do a masters/post-grad degree at an overseas uni...(I'll leave that debate to the other threads!).
