NDA

Not for all. Test youeself through online tests to see whether you need guidance. Written exam is fairly easy to crack it is the SSB that frets aspirants. You can opt for short term SSB coaching if you are not confident about it.

You can register at UPSC website though NDA protal.

It depends upon the entry for which you are applying for . Moreover 90% is good score to be eligible for all entries but do check whether the entry contain written exam or not.

Minerva Academy
Balnoi Academy
Major Kalsi Classes
Cornel Dagar Classes, etc. are the best coaching in India.

SSB has nothing to do with you medical condition. If you have the quality, then you will be recommended then later, you medical board will decide whether you will join academy or not.

Right now, National Defense Academy is only for boys but you may get a chance in the future.

You need to apply for NDA in UPSC website. Fill up the application form and add preference to Air force. After that, when you get admit card you have to give entrance exam of NDA. If you shall pass, then you will got call letter from SSB. When you crack SSB, then you will be directed to medical test. If they declared you medically fit then you will get joining letter to NDA.
Best of luck.

If you completed your 12, then you can apply. But for applying for air Force or navy though NDA you need PCM in 12 as well!

If you join NDA for Air Force and you have to go for flying branch only.

Checkout this Answer on Shiksha
https://ask.shiksha.com/how-can-i-join-nda-what-is-the-best-way-for-the-same-qna-4182280?referenceEntityId=4185170.