Start with 6th - 12th NCERTs.
Read newspapers - The Hindu/Indian Express (Never miss editorials)

If you continue this for 1 year alongwith your B.Tech. At the time of your graduation, you will be much ahead of all those preparing with you.

If you want you can start your preparation side by side your B.Tech programme. 

You can make your time-table according to your comfort. So, that you can balance both the things. 

Before starting your preparation you should be aware of the complete syllabus,  exam pattern and important topics for your preparation. So, that you can prepare accordingly.

All the Best!!