All entries after class 12 are compulsorily PERMANENT COMMISSION. As a Permanent Commission Officer you will be required to serve for a minimum period of 20 years from the date of commission. Though you may leave before 20 years also but you will not be eligible for life long pension and other ex-service men facilities which will be applicable if you retire after 20 years.

After 12th you need to appear for NDA Entrance exam to go for Indian air force. You can check the eligibility for NDA with given below link.

NDA Eligibility