To study in Australia, you'll need to apply for both admission to an institution and also for a student visa from the Australian Government. Application deadlines vary from course to course, so please check any specific deadlines on the website of the university.
The academic year in Australia starts in February!
Semester 1 runs from February to June, semester 2 from July to November.
Almost all courses can be started in the first semester, some can also be started in the second.