For this course which site should be taken

Hello Bhuvan.

Well it depends because mobile designing is further divided into two parts.

1.Hardware mobile designing.

2.software mobile designing.

If you are interested in physical designing of mobiles then you should go for ece branch.

If you are interested in application development then you should go for  cse branch.

Courses can be taken like computer science or information technology.
For which it will be easier to understand the mobile design concepts and grow the skills.
Even if you take any other course like EE or ECE you can even learn mobile design but the matter is you have to learn from the basics.
Mobile design is a field which is related to system design.
Choose technical branches it will be easier to you.
Hope it helps!

Hello Bhuvan
As per my knowledge ; I think you are talking about designing and structure ( means shape ; size ; look ) of the mobiles .
For this you should go with mechanical engineering ( although mobile designing will not be taught to you ) ; but designing software like cad ; solid works etc. you can learn and will be taught to you through which you can design anything .
Good luck