hi prince

let me start with the last question first. NDA and IIT are both very reputed streams. But they are vastly different and require different skills and mindsets to succeed. You need to find out what works best for you.
If you need a more detailed discussion about skills and qualities required for each area, please write back and I will be happy to elaborate further.

As for preparing well for 12th, the following might help.
1. prepare a plan for the year
2. make daily time table. make sure that you do not overburden yourself. it should be something that you can follow for long time. leave gaps for relaxation and recovery
3. practice practice and more practice. that is probably the only sureshot way to success
4. make sure to work and study with a few good friends. this way you might be able to help each other out
5. do not try out 10 different books. find one or two good books and stick to them.

hope this was of some help