Science

No one can actually tell you that how you have to chose your stream . It is completely your decision. It depends on your interest and will. You can choose any of the career in these subjects. All of these have immense scope.
However if you are willing to go in anyone of them then you can consider various factors like jobs available, salary offered etc.
In engineering you can get job faster than in medical. But nowadays it is difficult to get a job as the number if engineers is huge. Medical filed almost guarantees you a job but you have to work harder and for longer than in engineering. For computer science you can do engineering or may be masters. Maths can be chosen as a separate career in itself. You can do masters and go for teaching line.
At the end it depends all on you and your interest. You have to decide what is good for you and how much you can work for it. Your hard work and willingness will matter more than the course you choose.
Hope it helped;)

As suggested earlier, there are no important topics for jee. We never know which topic is given the highest weightage of marks until we have a look at the question paper. but there sure are some ways to avoid the sea of syllabus and get into an IIT. to play SMART.
you must be smart enough to pick the easiest yet relatively important topics to gain the maximum marks possible.have a look at previous papers and analyse the importance of topic.
and it is always a safe bet to have an average in all subjects than one or two favourites.
Lets start with out subjects.
1)MATHS - probability, integral calculus, coordinate geometry, straight lines, complex numbers, matrices and determinants. each chapter needs one specific skill to be an expert in that area.
for eg, probability needs good analysis of question, coordinate geometry needs good imagination of the bigger picture of the problem. give more time to the topic you feel would fit your style.
2)PHYSICS- rotational motion, oscillations and waves, laws of motion, current electricity, magnetism, modern physics. and then there are little less important but much easier topics compared to the former like modern physics, optics, and thermodynamics. so better invest time on these easier topics than nightmare like rotational motion. magnetism and current electricity are important and closely related topics. knowledge on one topic highly coordinates for the second.
3)CHEMISTRY (good knowledge of chemistry can save a lot of your time in exam) - coordination compounds(must do-most imp topic), chemical thermodynamics(similar to the one in physics), chemical kinetics, chemical equilibrium, S and P block elements, solid state and when it comes to inorganic chemistry, good knowledge of NCERT is a must(saves a lot of time too).
ALL THE BEST for your exam.