First decide whether you want to pursue engineering or medical, as for the former PCM combo in 12th recommended, while for medical, it's PCB.
Of course, you could take PCM-B to have a shot at both, but that's your decision.
You can sort that out by taking aptitude tests, seeking career counseling from teachers, parents, institutes etc.
If you are keen on engineering, then read on: (as I dont know what's to be done for medical)
Studying for the IITJEE is no walk-in-the-park. You need to study with full dedication for atleast 5-6 hours a day to have any hope of even comprehending the exam.
As you are in 10th, I'll advise not to concentrate on IITJEE now as it is fruitless. You are definitely better off with revision of your 10th standard subjects than adding another subject to the list.
I'm saying this as the portion for such exams is far more in depth from even 12th CBSE text books (they'll only help in AIEEE, not much in JEE). To study from the JEE (or any competitive exam), you'll need other books, which you can only understand once you start with 11th and join a coaching institute (if required) at that time. It's a waste of time really to think of entrance exams, especially since you are just in 10th.
My advice is to just keep your basics sharp, ensure you remember all that you have learnt in 10th as that should provide a solid base for engineering entrance exams (like JEE and AIEEE) once you reach 11th.
Just my 2 cents...
I will advice you to ask you teachers/parents as to how to go about though.