For join Indian Air Force after 12th you have to qualify for NDA exam. It is conducted by UPSC.
Eligible for NDA exam you must be between 16.5 and 19.5 years of age.
Candidates who will qualify the written test of NDA 2017 will be called by Services Selection Board for Intelligence and Personality Test
To know in detail you can refer below link
Get link http://www.upsc-2017.in/nda-exam-date/