It's perfectly fine if you have not done unit 1 or 2 psychology - the only thing that's required from 1/2 knowledge in 3/4 psych is the RESEARCH METHODS component. If you print off the study design for units 3/4 psych, should tell you all that you are required to know for research methods. Learning research methods is the most tedious bit (you will literally hate it initially), but I gradually started to like the formulaic aspect of it (when it comes to stuff like SA or Extended answer question in practice exams).
I used the Grivas psych book. I ALSO used the oxford 3/4 psych book (NB there is no way in hell to access the solutions for chapter and review questions so don't bother trying for this book...just ask your teacher if you want someone to check your answer). Read both books - heightens your understanding.
Yeah heaps of people say checkpoints is a waste of money. I disagree. Being an organized person, I liked how the questions from past VCAA exams were arranged topic by topic. So when the time came for me to start doing some exam style practice questions, I could just go "hey, I'm weak at memory...let me do some questions from there".
I started off the year attempting to use all sorts of resources (NEAP books, TSFX notes from lecture I attended before the mid year exam), but I can honestly tell you now that the only resouces that I ACTUALLY used was cambridge checkpoints and my two textbooks. I found the TSFX lecture honestly a waste of time and didn't really find the notes helpful. I suppose it varies from person to person, but I found that I learnt a whole heap more when I just studied by myself. As for the NEAP study guides, they do have some good questions. Like I said, I didn't really use it because my teacher supplied me with a lot of other random sheets of practice questions (so obviously I couldn't fit in enough time to do the NEAP questions and they became unrequired). Definitely recommend you buy the NEAP though, just in case your teacher doesn't give you enough extra material to practice with. It will come in handy then.
Where to get NEAP? Try ordering it online or buy/place it on order from Links bookshop.
Tutor is absolutely not necessary for psych. Whatever questions you have should be able to be answered by your teacher. If not, message people on Atar notes for help. I, for example, emailed my teacher on a regular basis (I had heaps of question about the course content) and she answered all of them.
Practice exams...start them end August/early September. I only did about 5 for the end of year exam. Some people say it's best to do 15. It really depends on you. My belief...after having done 5 I recognized the format of the exam really easily. I developed specific template answers to questions which were pretty much asked in the same way year after year. After I had memorized these template answers, I was comfortable and knew that I'd be wasting my time if I did any more practice exams. I cannot stress how important it is to really develop these template answers for those repetitive questions (you'll see the question in the exam and be like "I've answered that a million times!" so it'll save you time in the exam).
Psych's a broad subject. Heaps of content. People say it's easy, but it's obviously not easy to get 48+ for any subject. I can't tell you how difficult it would be (that's a pretty subjective question). I found it tough but very enjoyable