CBSE 10th

Here are the eligibility criteria for CAT 2017:
The candidate must possess a Bachelors Degree with at least 50% marks or equivalent CGPA (45% for SC, ST and PWD/DA category).
Percentage obtained in Bachelors degree would be calculated on the basis of practice followed by the concerned university/institution. In case of grades/CGPA, the conversion would be based on the process defined by the concerned university/ institution.
In absence of any percentage conversion scheme, candidates CGPA will be divided by maximum possible CGPA and multiplying the result with 100.
Candidates appearing for the final year of Bachelors degree/equivalent qualification are also eligible to apply for CAT.

It depends on many factors. A list of the following will give you a clear picture.
1.Interest: As you pointed out you want to do CS (Computer Science) if you are really sure about it. I would say go for diploma in a very good college. The quality of Diploma education is not great as it used to be 10-12 years ago. so, if diploma- choose a good college.
2. College: As I told you if you want to do a Diploma, good college is a must, track record, labs, faculty etc. must be taken into account.
* If you take diploma, you will get an entry in a good Engineering college at the state level. But, you will gravely miss out on IITs, NITs, and other national level colleges. which is a big minus for an aspiring Engineer *
3. Your State's Scenario: Some states in India don't at all give much importance to diploma education. instead +2 is preferred. Else, In a state like Andhra Pradesh, Maharashtra, Diploma students have a good number of seats in Engineering colleges to get into. So, take those factors also into account.
4. your zeal and aptitude for CS: you need to have a zeal for coding, developing, I know many people regret their work after joining their jobs. It's not that fun being a coder if you don't have a real zeal for it. And you need to develop your skills to match the standards and be the best.
Try reading about coding, and what innate skills you should have.
The above are a few pointers which will help you out.
now, few facts about both diploma and +2
Diploma :you would be confined to your stream only. Later, after a year, if you feel that CS is not working for you, you can not change your choice. If your decision is mature and very well informed choice. Then, go for a Diploma.
+2 (11 and 12) :This is always a safe bet, you can have the choice of targeting IITs , NITs etc. or you can also explore other career options like BBA, B.Sc, B.Com.
Purely personal opinion: In my opinion going for 11, 12 is a good because you get some more time to explore your options with more insights and experience.If you find Mass communications or Law more interesting, you can have a chance, unlike Diploma. If you like After 11th if you still want to go for Engineering in CS, prepare well for JEE, state level Entrances, other univs entrance and go ahead.
Whatever you do, love it. Don't think about the money.
Interest really matters!
Don't regret.
All the best!

Hi Shivam,
The minimum eligibility criteria (educational qualification) for NDA examination is:
Educational Qualifications:
(I) For Army wing of National Defence Academy: 12th Class pass of the 10+2 pattern of School Education or equivalent examination conducted by a State Education Board or a University.
(ii) For Air Force and Naval Wings of National Defence Academy and for the 10+2 Cadet Entry Scheme at the Indian Naval Academy: 12th Class pass of the 10+2 pattern of School Education or equivalent with Physics and Mathematics conducted by a State Education Board or a University.
Candidates who are appearing in the 12th Class under 10+2 Pattern of School Education or equivalent examination can also apply for this examination. These are the conditions to sit for NDA examination in which case diploma might not be the right choice. For more details, please log on to the official website:
http://upsc.gov.in/