Don't worry, before Y11 I never did a practice exam either. For the Y10 English exam, my friend did 10 essays over a weekend just before the exam, whilst I did absolutely nothing but prepare a quotes sheet.
For your 3/4 you should definitely do at least 20 practice exams, all in one sitting and under exam conditions. Practice exams help you A LOT, I would go as far to say that they are essential. But make sure you do them only after you have fully developed your understanding of the course, when there is nothing more you can learn from the textbook. As for where you get them, your teacher will give them to you, or you can find some on the Internet (check out the VCAA website for the official ones). Also, be sure to check out the VCAA assessor's reports - they tell you common mistakes made on exams.
I wouldn't bother trying to do 20-30 practice exams for your 1/2s, I only did what my teacher gave me, which was about 5 every subject.
One warning I have to give you is that for English, DO NOT write the essays under exam conditions too early. Otherwise your essays will be bland since you didn't spend much time thinking about the content.
Good luck.