Dear student. Good that you are showing intrest in  IAF.Actually there are lot of opportunities in IAF B.Tech or B.Sc is not necessary for that. You can appear immediately after intermediate or 10 +2 . Aspirants can join IAF as Flying officer/ Branch, Technical Branch, Ground Branch/Staff in IAF.

To join IAF after 10+2 you have to crack NDA (National Defence Academy). Examination conducted by UPSC. Eligibility to appear NDA exam is 10+2 (PCM )with minimum 60%  aggregate.Age limit is 16 1/2years. to 19 years. You can also opt for TES (Technical Entry Scheme). If you are between 17 to 19 years and passed your 10+2 (PCM) with 70% or above you are supposed to pass a small entrance conducted by defense services.

If you want to join IAF after graduation and your dream is to become trainee pilot (Flying branch) , you can appear for CDCE .and If you wish to join Flying branch as a Air Wing  Senior Division 'C' certificate of National Cadet Corps appear for NCC Special Entry.Both the exams are conducted by UPSC.Even after your Graduation in engineering , you can pursue as a Technical Officer by clearing these exams. University Entry Scheme (UES) and  AFCAT. There are some opportunities after post graduation also. But be clear in IAF required physical stature is very important.

Hope it helps.

All the best