In order to join Air Force after class 12th you can apply for NDA. As per NDA eligibility:

The educational qualification: you should be passed 10+2(with physics and mathematics) to apply for the exam. The exam is conducted twice every year

Age limit: Your age limit should be 16 to 19 Years.

For further updated information you can go through with given below link.

https://competition.careers360.com/articles/nda-2018

Good Luck!