Normally this question is not considered before starting preparation for competitive exam. I think its important question, a small thought and direction can give you better results for your hard work.
My suggestion is as follow(though many may differ from this)
1 - Look at the course deeply
2 - Look at the weight-age of each topic in the test
3 - There are few topics which are used in other topics too, example for pre-engineering is integration and differentiation in Maths, better understanding of this is not only for good marks in maths but also for physics. If you are not good in these topics and you give more focus on physics you may face difficulties and may need to invest much more time then required.
4 - After above analysis look at your interest, subjects and topics you like most and topics you are afraid most of.
5 - Its also advisable to give more emphasis on topics which have higher weightage.