No point in trying to aim for first when you aren't doing so well. What you should do, is aim to progressively move upwards, not just jump from nowhere to 1st.
For e.g
In physics, I was ranked 30ish out of 60 last term. I made the decision at beginning of HSC to have a goal for each term.
I set it out like so,
At end of first term, I want to be in the top half of the year, i.e top 30.
End of 2nd term I want to be in the top 15
By end of 3rd term, I want to be in the top 10 (by this point, you should already be happy)
If you are persistent bout that 1st, then 4th term is where you take it. Trials are usually worth the most towards ur HSC, and it comes at a time when you want to move up in ranks...HOW HANDY!!
now...all you have to do is ensure you study well.
It worked for me. In fact, It's worked so well, I was only planning to be top 20 this term, but at the moment I am ranked 2nd. You just have to set up these goals, and work REALLY REALLY REALLY hard to achieve them. I mean, it's hard for the first few weeks, because you're making a change to your study pattern. For me, I began studying everyday for physics, and by the end of the first 2 weeks, I was beginning to lose motivation. Then I took the weekend off and thought about my decision. I decided to push through, and have come through with no problems since.
So the moral of the story is:
STAY MOTIVATED
CONSISTENCY IS THE KEY
SABOTAGE ALL ENEMY <--nah joking lol....but really, you need to study quite hard. Say 30 minutes EACH SUBJECT a day (that is, NOT YOU HOMEWORK AND ASSIGNMENTS, BUT PURE REVISION, PRACTICE PAPERS etc...) and that will be enough to get you a 99+ UAI for the HSC. It sound simple....but 99% of people cannot carry it out. Prove me wrong.