Hi Hemendra,
In IMA(Indian Military Academy),cadets are taught about different regiments in last phase of their training to make them know about it.
Cadets can select their regiment in which they want to go for but it also depends upon vacancies also.
Also, if your father served in the army, there is a chance of getting you in the same regiment.
Engineering graduates can go for some regiments only as they involve only technical work.
If a regiment got more applicants than its vacancies, then they select the cadets on the basis of merit and cadets who do not make into that regiment have to go to other regiment.
All the regiments in Indian Army has equal respect . So, you can go for any regiment because it's a thing of pride to be part of it.
It depends upon your performance also as merit is made and if you perform well you have great chance to get regiment of your own choice.

Best Wishes