You can join air force after clearing NDA examination,and the subsequent SSB that follows ,the SSB for Airforce spans over 6 (unlike 5 for other wings) days, as it also includes Pilot Aptitude Battery Test(PABT) this is a tricky part because once you fail in PABT you aren't allowed to appear for it again.

For more information about NDA, you can go through the below link -

NDA 2017

Please go through the below given detail -
Air Force, Navy and Naval Academy of National Defence Academy: Candidates applying for Air Force, Navy and Naval Academy must have passed class 12/HSC in 10+2 pattern of School Education with Physics and Mathematics conducted by a State Education Board or a University.

For more information, you can go through given below article -

NDA Eligibility Criteria 2017

Hope this helps!!!