Hi,

In order to persue ur btech you need to appear for JEE exam. This exam is conducted in two stages. JEE mains nd JEE advance. 

Mains is for admission into NITs/IIITs/GFTs

Advance is for admission into IITs. 

To be eligible for applying for jee you need to qualify ur std 12th with PCM.

Your mains consists of two papers.

Paper 1 for b.tech/b.e  having mcq based question from maths, physics nd chemistry. Total marks 360(90x4). It will be computer based

Paper 2 is for b.arch/b.planning. Total marks 390 having questions from maths, aptitude and drawing skills. It will be partly CBT amd partmy pen n paper. 

Once you qualify mains you shall be eligible for counselling for NITs/IIITs/GFTs and also for appearing for jee advanced.

For admission into NITs/IITs/GFTs you need to have a 75% in 12th boards.

And for IIT you need to appear for jee advance.

Jee advance will be CBT. It ll have two papers both being mandatory. In each paper questions will be asked from maths, physics nd chemistry. 

Qualifying advance makes u eligible for getting into IIT. 

You can also refer to the following link:

https://engineering.careers360.com/articles/jee-advanced-exam-pattern

Hope it helps. 

All the best.