You can fill the online NDA application forms from the official website of the Union Public Service Commission (UPSC) at the time of availability of it.

Important dates
Starting date for Submission of Online Application: 21-12-2013.
Last date for Submission of Online Application: 20-01-2014.
Date of Written Examination: 20-04-2014.

As you can see that all dates are over, so currently the application form is not available at the official website of it.

Other details of the examination:

Required qualification:
The applicant should be 12th class pass of the 10+2 pattern/ equivalent with Physics and Mathematics for Air Force and Naval Wings of National Defence Academy and for the 10+2 Cadet Entry Scheme at the Indian Naval Academy.

Selection procedure:
The selection will be based on written test followed by Psychological Aptitude Test and Intelligence Test, SSB Interview.