Well the subjects you THINK you may be good at in form 5 might not be what you WILL BE good at in form 6 since it is a different ballgame all together
If you compare it with the old syllabus, the new syllabus is much more watered down really. It will be definitely more competitive since the average students will be playing at the same field as those above average. All students are basically on the same ground right now. So the key really is to work hard since day 1
As for what subjects you plan to take, it depends on what your aim really is in the future, and your capacity as well. Physics, though one might plan to pursue a career in life sciences and might disregard it, is extremely useful in the basis of understanding and conceptualizing while consolidating your facts. Having said that, one who plans to pursue engineering will find that knowledge in biology gifts one with foresight on incorporating technology and life.
A little more extra information is no harm; what kills is knowing too little !!
Think carefully of what you choose. Most would think that form 5 is where life changes, that an A star star indicates how highly intelligent one is. Honestly, that is worth nothing AT ALL. What puts you into a real challenge is going through form 6. Choose wisely.
As for tuition, it is extremely important that you have an excellent tutor who is able to condense crucial information in a short time into your brain. Remember that you will still have to study up quite a vast bit of information albeit the new syllabus
These sites may be able to help you in searching for a tuition teacher aside from providing additional information on what STPM is all about.
Personally, I would recommend Mr Lingam. He is both at Kasturi KL and has started on his own in Kelana Jaya. If you do study under him (ESPECIALLY CHEMISTRY !!), you will realize how much more information there is out there in this world.
All the best in your future endeavours
If you need to clarify some things, do not hesitate to reach me
An ex STPMer's two cents' worth