Dear Bhavesh,

It depend upon the exam or stage you applied for, you can go to any defense force just after 12th or after graduation.

1. After 12th

You have to appear for NDA and there syllabus is same for the the branches of defense like Army, Navy and Airforce. After that they call you for SSB then for medical if are recommended they your name will come on the merit list after that you can join any of the preferred force ( in the application form you need to clear that which force you want to join ).

2. After graduation

There are different ways to join defense after graduation. For CDS you have to complete the same syllabus for Army, Navy and Airforce to get the admission in IMA, INA, AFA which is English, General Knowledge and Elementary Mathematics. But you need engineering degree for Navy and Airforce and any degree for Army. There is separate exam for Airforce called AFCAT where the syllabus is English, General Ability, Numerical Ability, Reasoning and military aptitude.

So you see there is not much difference in the syllabus of any defense exam, you have to prepare for one syllabus and eligible for all.

Best of Luck!!!